Name: finding ph in acid mixtures
Uploaded: 2025-05-22T09:06:25.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Chemistry

Matter and Its Interactions

9-12.C.1.6.e

Mixtures of Acids

In una miscela di acido forte e debole, l'acido forte si dissocia completamente, aumentando significativamente la concentrazione di ioni H3O+, mentre l'acido debole si dissocia solo parzialmente.
Il principio di Le Châtelier spiega che l'acido più forte sopprime la dissociazione dell'acido più debole spostando l'equilibrio verso i reagenti.
Si consideri una miscela di 0,15 M HCl e 0,30 M HCN. L'acido forte produce una concentrazione di H3O+ di 0,15 M, rendendo trascurabile la concentrazione di H3O+ dell'acido debole.
Quindi, il pH in una miscela di acidi è prevalentemente dettato dalla concentrazione dell'acido forte.
Allo stesso modo, quando è presente una miscela di due acidi deboli di quantità uguali, l'acido relativamente più forte è il principale determinante del pH.
Ad esempio, in una miscela di HF e HCN, l'HF sarà il principale determinante del pH della miscela, poiché ha un Ka di 3,5 × 10−4 che è quasi un milione di volte superiore al Ka dell'acido cianidrico.

Nessun conflitto di interessi dichiarato.

miscela di acidi

Il pH di una soluzione contenente un acido può essere determinato utilizzando la sua costante di dissociazione acida e la concentrazione iniziale. Se una soluzione contiene due acidi diversi, il suo pH può essere determinato utilizzando uno dei diversi metodi a seconda della forza relativa degli acidi e delle loro costanti di dissociazione.

In una miscela di acidi forti e deboli, l'acido forte si dissocia completamente e diventa una fonte di quasi tutti gli ioni idronio presenti nella soluzione. Al contrario, l'acido debole mostra una dissociazione parziale e produce una concentrazione trascurabile di ioni idronio. L'elevata concentrazione di ioni idronio prodotti dall'acido forte riduce ulteriormente la dissociazione dell'acido debole. Secondo il principio di Le Chatelier, ciò accade: “Quando un sistema chimico in equilibrio viene disturbato, il sistema si sposta in una direzione che minimizza il disturbo”. Gli ioni idronio in eccesso prodotti dall'acido forte disturbano l'equilibrio e quindi la reazione si muoverà nella direzione opposta fino a quando non viene stabilito l'equilibrio. Ciò porta ad una diminuzione della dissociazione dell'acido debole. A causa di questa diminuzione, il pH di una miscela di acido forte e debole può essere calcolato solo dalla concentrazione dell'acido forte. Ad esempio, il pH di una miscela con una uguale concentrazione di acido cloridrico (HCl), un acido forte, e acido formico (HCHO2), un acido debole, può essere determinato solo dalla concentrazione di HCl. Se la concentrazione dell'HCl nella miscela è 0,0020 M, il suo pH può essere calcolato come segue.

pH = -log(0,002) = 2,7

Qui, la concentrazione di ioni idronio prodotti da HCHO2 e l'autoionizzazione dell'acqua è trascurabile e quindi può essere ignorata. 

Una miscela di due acidi deboli con diverse costanti di dissociazione

In una miscela di due acidi deboli, il pH di una miscela sarà determinato dall'acido più forte se la sua costante di dissociazione è significativamente più alta di quella dell'acido più debole. Ad esempio, in una miscela con una uguale concentrazione di acido nitroso (HNO2) e acido ipocloroso (HClO), l'HNO2 sarà il principale determinante del pH della miscela poiché la sua Ka (4,6 × 10−4) è circa 10.000 volte superiore al Ka (2,9 × 10−8) di HClO. Secondo il principio di Le Chatelier, l’HClO mostra una minore dissociazione in presenza di HNO2.

Miscela di acidi

In una miscela di acido forte e debole, l'acido forte si dissocia completamente, aumentando significativamente la concentrazione di ioni H3O+, mentre l'acido debole ...

Education

Didattica

JoVE Core

Analytical Chemistry

Chimica Analitica

Unit 1

Unità 1

Acid–Base Titration

Titolazione acido-base

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

finding ph in acid mixtures

Finding pH in acid mixtures depends on the acids present and their dissociation constants. When a solution contains two acids, the pH can be found by choosing the method that matches their relative strengths. The key idea is that the stronger acid often controls the hydronium ion level, which is the amount of H3O+ in solution.

In a mixture of a strong acid and a weak acid, the strong acid dissociates completely. It becomes the main source of hydronium ions in the solution. The weak acid dissociates only partly, so it adds a very small amount of hydronium ions. The high hydronium ion concentration from the strong acid also suppresses the weak acid.

This effect follows Le Chatelier’s principle. When an equilibrium system is disturbed, it shifts to reduce the disturbance. In this case, the extra hydronium ions from the strong acid push the weak acid equilibrium in the reverse direction. As a result, the weak acid dissociates less. Because of this, the pH of a strong and weak acid mixture can be calculated from the strong acid concentration only.

For example, a mixture with equal concentrations of hydrochloric acid, HCl, and formic acid, HCHO2, can be treated using the HCl concentration alone. If the HCl concentration is 0.0020 M, then pH = −log(0.002) = 2.7. The hydronium ions from HCHO2 and the autoionization of water are negligible here and can be ignored.

A mixture of two weak acids works differently. If one weak acid has a much larger dissociation constant, Ka, than the other, it will usually determine the pH. For example, in a solution with equal concentrations of nitrous acid, HNO2, and hypochlorous acid, HClO, HNO2 is the main factor because its Ka value, 4.6 × 10−4, is about 10,000 times higher than the Ka of HClO, 2.9 × 10−8. Le Chatelier’s principle also explains why HClO dissociates less in the presence of HNO2.

Finding pH in Acid Mixtures

acid&#8211;base titration: overview

Titration Curves and pH Changes

Acid-base titration uses a known acid or base to find the concentration of an unknown solution. A base can titrate an acid, or an acid can titrate a base. As titrant is added little by little, the pH changes in a predictable way. The pH change is shown on a titration curve. A titration curve plots how the pH of the solution changes as more titrant is added. Scientists use this curve to follow the progress of the titration and to identify the endpoint, which is the point used to stop the...

titration of a strong acid with a strong base

titration-of-a-strong-acid-with-a-strong-base

pH Changes in Strong Acid-Base Titration

pH changes in a strong acid-strong base titration depend on the amount of leftover hydronium or hydroxide ions. A strong acid such as hydrochloric acid fully dissociates in water. That gives the solution hydronium ions and chloride ions, so the starting pH is low. When sodium hydroxide is added, it neutralizes hydronium ions. As more base is added, the concentration of hydronium ions falls and the pH rises. At the equivalence point, the hydronium ions have been neutralized and the solution is...

titration of a weak acid with a strong base

titration-of-a-weak-acid-with-a-strong-base

Weak Acid Titration Curve and pH Stages

Weak acid titration with a strong base changes pH in clear stages. The acid in this example is acetic acid, and the base is sodium hydroxide. Each stage uses a different pH calculation method. At the start, the pH of the weak acid is found with its dissociation constant, Ka, and an ICE table. An ICE table tracks the initial, change, and equilibrium amounts of the acid. This step describes the acid before much base is added. As sodium hydroxide is added, a buffer forms. A buffer is a solution...

titration of a weak base with a strong acid

titration-of-a-weak-base-with-a-strong-acid

Ammonia Titration Curve with HCl

Ammonia titration with hydrochloric acid shows how a weak base behaves as acid is added. The curve is the mirror image of a weak acid titrated with a strong base. In this case, ammonia acts as the weak base, and hydrochloric acid acts as the strong acid. The starting pH of 50 mL of 0.1 M ammonia is found with an ICE table and the Kb value. That calculation gives an initial pH of 11.11. As hydrochloric acid is added, the pH begins to fall. After 25 mL of 0.1 M hydrochloric acid is added, the...

titration of a weak acid with a weak base

titration-of-a-weak-acid-with-a-weak-base

Finding the Endpoint in Weak Acid–Base Titration

Weak acid and weak base titration is used to follow how pH changes as base is added. In this kind of titration, both the acid and the base dissociate only partly. The titration curve shows a slow, gradual pH change instead of a sharp jump. A common example is the titration of acetic acid with ammonia. During the reaction, these two species form ammonium acetate and water. Because the pH changes so gradually, there is no simple indicator that shifts color sharply at the equivalence point. One...

solution composition during acid/base titrations

solution-composition-during-acidbase-titrations

Acetic Acid Buffer Changes in Titration

Acetic acid buffer changes during a weak acid–strong base titration. When acetic acid is titrated with sodium hydroxide, the reaction forms water and the conjugate base of the acid, sodium acetate. The mixture of acetic acid and sodium acetate acts as a buffer, which helps show how solution makeup shifts during the titration. The relative amounts of acetic acid and acetate ions are tracked with α values. Here, α0 represents the equilibrium concentration of acetic acid, and α1 represents the...

mixtures of acids

Finding pH in acid mixtures depends on the acids present and their dissociation constants. When a solution contains two acids, the pH can be found by choosing the method that matches their relative strengths. The key idea is that the stronger acid often controls the hydronium ion level, which is the amount of H3O+ in solution. In a mixture of a strong acid and a weak acid, the strong acid dissociates completely. It becomes the main source of hydronium ions in the solution. The weak acid...

titration in nonaqueous solvents

Nonaqueous Solvents for Sharper Titration Endpoints

Nonaqueous solvents can make acid-base titrations easier to read by giving sharper endpoints. Most acid-base titrations are done in water, but water can compete with weak acids or bases for proton donation or acceptance. That competition can blur the titration curve and make the endpoint less clear. Water also changes the partial ionization of weak acids and bases. For example, water can accept a proton from acetic acid to form hydronium and acetate ions. The hydronium ion is a stronger acid...

titration of polyprotic base with a strong acid

titration-of-polyprotic-base-with-a-strong-acid

Carbonate Buffering in Acid Titration

The titration of sodium carbonate with hydrochloric acid shows how a polyprotic base reacts in stages. A polyprotic base can accept more than one proton, so its titration curve has two equivalence points. These points mark where one form has been fully changed into the next. At the first equivalence point, carbonate ions are completely converted to bicarbonate ions. The region before this point is the carbonate/bicarbonate buffer. In this buffer region, both forms are present in the solution. <br...

titration of polyprotic acids with a strong base

titration-of-polyprotic-acids-with-a-strong-base

Polyprotic Acid Titration Curve Features

Polyprotic acid titration curves show how acids with more than one ionizable proton react with a strong base. Each proton is released in a separate step. Each step has its own dissociation constant, or Ka, and each later Ka is weaker than the one before it. When a polyprotic acid such as sulfurous acid is titrated with sodium hydroxide, the base first neutralizes the first ionizable proton. This forms an intermediate species, such as hydrogen sulfite ions. The first part of the curve looks...

composition of polyprotic acid solutions as a function of ph

composition-of-polyprotic-acid-solutions-as-a-function-of-ph

Polyprotic Acid Titration Fractions at Each pH

Polyprotic acid titration shows how the main species in solution change as pH changes. In a compound of the form H2M, there are two ionizable protons. That means the titration with a base produces two equivalence points on the curve. As base is added, three species can be present in the solution: H2M, HM−, and M2−. Their amounts are shown as fractions called α0, α1, and α2. These alpha values track how much of each form is present at different stages of the titration. At the start of the...

buffers: overview

Buffers and pH Stability

Buffers help keep the pH of a solution stable when small amounts of acid or base are added. They are made from a weak acid and its conjugate base, or from a weak base and its conjugate acid. This balance lets the solution resist large pH changes until the buffer capacity is exceeded. One common buffer uses acetic acid and sodium acetate. Here, CH3COOH acts as the weak acid and CH3COONa provides its salt. Another buffer uses ammonia and ammonium chloride, written as NH3 and NH4Cl. These...

buffers: buffer capacity

Buffer Capacity and pH Resistance

Buffer capacity describes how well a buffer resists changes in pH. It is a quantitative measure, so it tells how much acid or base a one-liter buffer solution can take before its pH shifts by 1 unit. In the equation, beta is the buffer capacity, while Ca and Cb represent the moles of acid and base added. The term dpH means the change in pH. A graph of pH versus the moles of base added to a weak acid with a pKa of 5 shows how buffering works as more base is added. Taking the derivative of that...

leveling effect

How Solvents Limit Acid and Base Strength

The leveling effect explains how a solvent can limit acid and base strength in solution. In acid-base chemistry, the solvent sets a ceiling on how strong an acid or base can act. Stronger acids or bases may react with the solvent itself instead of with the intended compound. When a base is stronger than the conjugate base of the solvent, it deprotonates the solvent first. A conjugate base is the species left after an acid loses a proton. This reaction keeps going until the stronger base is...

key terms and concepts

Learn how pH is determined in acid mixtures, from strong acid and weak acid combinations to two weak acids with different Ka values.

Finding pH in Acid Mixtures | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

1. Use the periodic table as a model to predict the structure and properties of atoms and elements.

9-12.C.1.1

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

a. Assess the merits and limitations of historic and modern atomic models pertaining to the presence, position, mass, and charge of subatomic particles.

9-12.C.1.1.a

b. Develop and use models of an element's subatomic particles to compare and contrast its atoms, ions, and isotopes.

9-12.C.1.1.b

The Wave Nature of Light

The nature of light has been a subject of inquiry since antiquity. In the seventeenth century, Isaac Newton performed experiments with lenses and prisms ...

The Electromagnetic Spectrum

The electromagnetic spectrum consists of all the types of electromagnetic radiation arranged according to their frequency and wavelength. Each of the ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

electron-configuration-of-multielectron-atoms

Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

c. Analyze and interpret data to identify or describe an element based on its number of protons, its relative abundance of isotopes, its organization and placement of electrons, and its light emission spectrum. Examples: average atomic mass, electron configuration, orbital notation, noble-gas notation, photoemission spectroscopy

9-12.C.1.1.c

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

surface-tension-capillary-action-and-viscosity

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded ...

Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

d. Ask questions to determine the relationship between an element's physical and chemical properties and its position on the periodic table. Examples: density, melting point, chemical hardness; reactivity, chemical stability, oxidation state

9-12.C.1.1.d

e. Construct explanations of how periodic trends can be used to predict the properties of elements. Examples: atomic radii, ionization energy, electronegativity

9-12.C.1.1.e

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

stoichiometry-product-yield-and-limiting-reactants

Stoichiometry, Product Yield, and Limiting Reactants

Chemical equations represent how a chemical reaction proceeds from reactants to products through physical or chemical change using chemical formulas.
...

4. Use stoichiometric ratios to support the claim that atoms, and therefore mass, are conserved during chemical reactions.

9-12.C.1.4

formula-mass-and-mole-concepts-of-compounds

Formula Mass and Mole Concepts of Compounds

Formula Mass of Covalent Compounds

Chemical formulas represent the elemental makeup of substances. For covalent compounds, the formula represents the ...

experimental-determination-of-chemical-formula

Experimental Determination of Chemical Formula

The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most concise way of representing this elemental makeup. ...

a. Quantitatively apply the concepts of the mole and Avogadro's number to conceptualize and calculate percent composition and empirical or molecular formulas of common compounds.

9-12.C.1.4.a

b. Use mathematical representations of the mole concept to solve reaction stoichiometry problems, involving mole-to-mole conversions, mass-to-mole conversions, and mass-to-mass conversions.

9-12.C.1.4.b

c. Use mathematical models to reveal the relationships among the theoretical, actual, and percent yields of chemical reactions.

9-12.C.1.4.c

d. Qualitatively and quantitatively determine the limiting reactant when given the masses of all reactants.

9-12.C.1.4.d

definition-measurement-pressure-atmospheric-pressure-barometer

Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

Gas pressure is caused by force exerted by gas molecules colliding with the surfaces of objects. Although the force of each collision is very small, any ...

gas-laws-boyle-s-gay-lussac-charles-avogadro-s-and-ideal-gas-law

Gas Laws: Boyle's, Gay-Lussac, Charles', Avogadro's, and Ideal Gas Law

Through experiments, scientists established the mathematical relationships between pairs of variables, such as pressure and temperature, pressure and ...

applications-of-the-ideal-gas-law-molar-mass-density-and-volume

Applications of the Ideal Gas Law: Molar Mass, Density, and Volume

The volume occupied by one mole of a substance is its molar volume. The ideal gas law, PV = nRT, &nbsp;suggests that the volume of a given quantity of gas ...

mixtures-of-gases-dalton-s-law-of-partial-pressures-and-mole-fractions

Mixtures of Gases: Dalton's Law of Partial Pressures and Mole Fractions

Unless individual gases chemically react with each other, the individual gases in a mixture of gases do not affect each other&rsquo;s pressure. Each gas ...

chemical-stoichiometry-gases-using-ideal-gas-law-to-determine

Chemical Stoichiometry and Gases: Using Ideal Gas Law to Determine Moles

Chemical stoichiometry describes the quantitative relationships between reactants and products in chemical reactions.

In addition to measuring quantities ...

basic-postulates-kinetic-molecular-theory-particle-size-energy

Basic Postulates of Kinetic Molecular Theory: Particle Size, Energy, and Collision

The ideal-gas equation, which is empirical, describes the behavior of gases by establishing relationships between their macroscopic properties. For ...

kinetic-molecular-theory-gas-laws-explain-properties-gas

Kinetic Molecular Theory and Gas Laws Explain Properties of Gas Molecules

The test of the kinetic molecular theory (KMT) and its postulates is its ability to explain and describe the behavior of a gas. The various gas laws ...

kinetic-molecular-theory-molecular-velocities-temperature-kinetic

Kinetic Molecular Theory: Molecular Velocities, Temperature, and Kinetic Energy

The kinetic molecular theory qualitatively explains the behaviors described by the various gas laws. The postulates of this theory may be applied in a ...

behavior-gas-molecules-molecular-diffusion-mean-free-path

Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many ...

real-gases-effects-intermolecular-forces-molecular-volume-deriving

Real Gases: Effects of Intermolecular Forces and Molecular Volume Deriving Van der Waals Equation

Thus far, the ideal gas law, PV = nRT, has been applied to a variety of different types of problems, ranging from reaction stoichiometry and empirical and ...

e. Use mathematics and computational thinking to perform gas stoichiometry calculations involving mass, volume, and number of moles at standard temperature and pressure (STP).

9-12.C.1.4.e

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solution Concentration and Dilution

The relative amount of a given solution component is known as its concentration. Often, though not always, a solution contains one component with a ...

Electrolyte and Nonelectrolyte Solutions

Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a ...

5. Obtain, evaluate, and communicate information concerning factors that affect solubility and the properties of solutions.

Finding pH in Acid Mixtures

Description

Transcript

Tags

More videos by Standards