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

Em uma mistura de ácido forte e fraco, o ácido forte se dissocia completamente, elevando significativamente a concentração de íons H3O+, enquanto o ácido fraco se dissocia apenas parcialmente.
O princípio de Le Châtelier explica que o ácido mais forte suprime a dissociação do ácido mais fraco, deslocando o equilíbrio para os reagentes.
Considere uma mistura de HCl 0,15 M e HCN 0,30 M. O ácido forte produz uma concentração de H3O+ de 0,15 M, tornando a concentração de H3O+ do ácido fraco insignificante.
Portanto, o pH em uma mistura de ácidos é predominantemente ditado pela concentração do ácido forte.
Da mesma forma, quando uma mistura de dois ácidos fracos de quantidades iguais está presente, o ácido relativamente mais forte é o principal determinante do pH.
Por exemplo, em uma mistura de HF e HCN, HF será o principal determinante do pH da mistura, pois tem um Ka de 3,5 × 10−4, que é quase um milhão de vezes maior que o Ka do ácido cianídrico.

Não há conflitos de interesse declarados.

mistura de Ácidos

O pH de uma solução contendo um ácido pode ser determinado usando sua constante de dissociação ácida e sua concentração inicial. Se uma solução contém dois ácidos diferentes, então o seu pH pode ser determinado utilizando um de vários métodos, dependendo da força relativa dos ácidos e das suas constantes de dissociação.

Numa mistura de ácidos fortes e fracos, o ácido forte dissocia-se completamente e torna-se uma fonte de quase todos os íons hidrônio presentes na solução. Em contrapartida, o ácido fraco apresenta dissociação parcial e produz uma concentração insignificante de íons hidrônio. A alta concentração de íons hidrônio produzidos pelo ácido forte reduz ainda mais a dissociação do ácido fraco. De acordo com o princípio de Le Chatelier, o que acontece é o seguinte: “Quando um sistema químico em equilíbrio é perturbado, o sistema muda numa direção que minimiza a perturbação”. O excesso de íons hidrônio produzidos pelo ácido forte perturba o equilíbrio e, assim, a reação se moverá na direção inversa até que o equilíbrio seja estabelecido. Isto leva a uma diminuição na dissociação do ácido fraco. Devido a essa diminuição, o pH de uma mistura de ácidos fortes e fracos pode ser calculado com base apenas na concentração do ácido forte. Por exemplo, o pH de uma mistura com igual concentração de ácido clorídrico (HCl), um ácido forte, e ácido fórmico (HCHO2), um ácido fraco, pode ser determinado apenas a partir da concentração de HCl. Se a concentração do HCl na mistura for 0,0020 M, seu pH pode ser calculado da seguinte forma.

pH = -log(0,002) = 2,7

Nesse caso, a concentração de íons hidrônio produzidos pelo HCHO2 e a autoionização da água é insignificantes e, portanto, podem ser ignoradas.

Uma mistura de dois ácidos fracos com diferentes constantes de dissociação

 Numa mistura de dois ácidos fracos, o pH de uma mistura será determinado pelo ácido mais forte se a sua constante de dissociação for significativamente superior à do ácido mais fraco. Por exemplo, em uma mistura com concentração igual de ácido nitroso (HNO2) e ácido hipocloroso (HClO), o HNO2 será o principal determinante do pH da mistura, pois seu Ka (4.6 × 10−4) é aproximadamente 10.000 vezes superior ao Ka (2.9 × 10−8) do HClO. De acordo com o princípio de Le Chatelier, o HClO apresenta dissociação diminuída na presença de HNO2.

Mistura de Ácidos

Em uma mistura de ácido forte e fraco, o ácido forte se dissocia completamente, elevando significativamente a concentração de íons H3O+, enquanto o ácido fraco apenas ...

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Acid–Base Titration

Titulação Ácido-Base

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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

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Finding pH in Acid Mixtures

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