Name: finding empirical and molecular formulas
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 37 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

experimental-determination-of-chemical-formula

Experimental Determination of Chemical Formula

化合物通常用 经验式或分子式来描述。这些式子提供了 有关所涉及元素的不同原子 数量的信息。但是这些式子是如何建立起来的呢？实验分析，如化合物的分解，被用于估计化合物中组成元素的 相对质量。然后用这些相对质量 来计算每种元素的摩尔数，从而确定化合物的分子式。例如，一种化合物的样品 经实验测定含有 43.64 克的磷和 56.36 克的氧。使用 摩尔质量作为转换因子，将从实验数据得到的相对质量 换算成 1.41 摩尔的磷 和 3.52 摩尔的氧。这些摩尔值，当被指定为元素的临时下标时，会产生化合物的一个伪公式。将这些摩尔值除以最小摩尔值 可得到大约 2.5 摩尔氧 与 1 摩尔磷的摩尔比，这与 化合物中元素的实际比例直接相关。如果其中一个商仍然是小数，那么将所有的数都乘以 最小的计数数，即得到下标的 最小整数比，这样就生成 P2O5 的经验式。化合物的分子式 可以根据其经验式 和摩尔质量或分子质量来确定。例如，经验式为 P2O5 的化合物 经实验测量其摩尔 质量为 283.89 克每摩尔。它的分子式是其经验式 的整数倍，而它的摩尔质量 是其经验式质量的整数倍。摩尔质量与经验式质量之比 得出公式单位的数量。经验式与公式单位数相乘 得出了分子式。因此，根据分子式 P4O10，该化合物被鉴定为十氧化四磷，或更为常见的是它的经验式 名称五氧化二磷。

化学式的实验确定

化合物的元素组成定义其化学特征，化学式是表示该元素组成的最简洁的方法。当未知某个化合物的公式时，测量其组成元素的质量通常是通过实验确定该公式的第一步。
 
 <h4>经验公式的确定</ h4>
 
 确定化合物化学式的最常见方法是首先测量其组成元素的质量。但是，化学式表示的是物质中原子的相对数，而不是原子的质量。因此，必须使用任何涉及质量的实验数据来获得化合物中相应的原子数。这是通过使用摩尔质量将每个元素的质量转换为其摩尔数来实现的。这些摩尔量用于计算可用于得出物质经验公式的整数比。
 
 考虑一种化合物样品，确定其含有1.71克碳和0.287克氢。相应的原子数是0.142摩尔的碳和0.284摩尔的氢。因此，该化合物可以由式C 0.142 H 0.284 表示。按照惯例，公式包含整数下标，可以通过将每个下标除以最小的下标（0.142）来实现。因此，该化合物的经验公式为CH 2 。下标“ 1”如果没有其他数字，则不写，而是假设。这可以是化合物的分子式，也可以不是。但是，需要额外的信息来进行该确定。
 
 作为第二实例，确定化合物样品含有5.31克氯和8.40克氧。用同样的方法得出ClO 3.5 的实验经验公式。在这种情况下，除以最小下标仍会在经验公式中保留小数点。要将其转换为整数，请将每个下标乘以2，保持相同的原子比，并得出Cl 2 O 7 作为最终的经验公式。
 
 <h4>从组成百分比导出经验公式</ h4>
 
 在化合物的百分比组成可用的情况下，它用于计算化合物中存在的元素的质量。由于百分比刻度为100，因此可以方便地计算重量为100克的样品中存在的元素的质量。获得的质量可用于得出经验公式。
 
 例如，假设一种气体化合物包含27.29％的C和72.71％的O。因此，质量百分比用分数表示：
 
 <img alt =“ Eq1” src =“ https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-01v2.png” style =“ height: 35px ;“ /> 
 
 <img alt =“ Eq2” src =“ https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-02v2.png” style =“ height:35px ;“ /> 
 
 碳的质量27.29 g对应于2.272摩尔的碳，氧的质量72.71 g对应于4.544摩尔的氧。因此，代表性的公式是C 2.272 O 4.544 。将每个下标除以2.272可提供经验公式：CO 2 。
 
 <h4>分子式的推导</ h4>
 
 确定组成一个共价化合物的单个分子的原子的绝对数量需要了解其经验式以及其分子质量或摩尔质量。这些量可以通过各种测量技术实验确定。例如，分子质量通常来自化合物的质谱图。
 
 通过将化合物的摩尔质量或分子质量与其经验式质量进行比较，得出分子式。顾名思义，经验公式质量是经验公式中表示的所有原子的平均原子质量的总和。如果将物质的已知摩尔质量除以经验公式质量，则得出每个分子的经验公式单位数（ n ）。
 
 <img alt =“ Eq3” src =“ https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-03v2.png” style =“ height: 40px ;“ /> 
 
 然后通过将经验公式中的每个下标乘以 n 来获得分子式，如通用经验公式A x B y ：
 
 <img alt =“ Eq4” src =“ https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-04v2.png” style =“ height: 17px ;“ /> 
 
 例如，将共价化合物的经验式确定为CH 2 O，并且其经验式质量约为30 amu。 如果确定该化合物的分子量为180 amu，则表明该化合物的分子中所含原子数是经验式中表示的原子数的六倍。
 
 <img alt =“ Eq5” src =“ https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-05v2.png” style =“ height: 35px ;“ /> 
 
 该化合物的分子由一个分子式表示，其下标比经验式大6倍：（CH 2 O） 6 = C 6 H 12 O 6 。
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas"> Openstax，化学 2e，第3.2节：确定经验和分子式。</a>

化合物的元素组成决定了其化学特性，而化学式是表示这种元素组成的最简洁方式。

Education

教育

JoVE Core

化学

Molecules, Compounds, and Chemical Equations

分子、化合物与化学方程式

molecules-compounds-and-chemical-equations

KEY TERMS AND CONCEPTS

关键术语与概念

finding empirical and molecular formulas

Chemical formulas show the elemental makeup of a compound. When a formula is unknown, scientists often start by measuring the masses of the elements in the sample. Those mass data are then used to find the simplest whole-number ratio of atoms, which gives the empirical formula.

To get from mass to atom ratio, the masses of the elements are first converted to moles using molar masses. Chemical formulas describe numbers of atoms, not masses, so this step is necessary. In one example, 1.71 grams of carbon and 0.287 grams of hydrogen become 0.142 moles of carbon and 0.284 moles of hydrogen. Dividing by the smallest value gives the empirical formula CH2, with the 1 omitted by convention.

Sometimes the numbers do not become whole right away. A sample containing 5.31 grams of chlorine and 8.40 grams of oxygen gives a tentative ratio of ClO3.5. Multiplying both subscripts by two keeps the same atom ratio and gives the whole-number empirical formula Cl2O7.

Percent composition can also be used to find an empirical formula. Because percentages are based on 100, it is convenient to imagine a 100-gram sample. For a gas that is 27.29% carbon and 72.71% oxygen, those masses correspond to 2.272 moles of carbon and 4.544 moles of oxygen. Dividing by the smaller amount gives the empirical formula CO2.

A molecular formula gives the actual number of atoms in one molecule of a covalent compound. To find it, both the empirical formula and the molecular mass or molar mass are needed. The empirical formula mass is the sum of the average atomic masses in the empirical formula, and dividing the molar mass by that value gives the number of empirical units per molecule, n. If the empirical formula is CH2O and the molecular mass is 180 amu, then n equals 6, so the molecular formula is C6H12O6.

Finding Empirical and Molecular Formulas

molecules and compounds

Atomic Elements, Molecules, and Bonds

Matter is made of elements, and elements are built from atoms. An atom is the smallest unit of an element that still keeps that element’s properties. A silver coin, for example, is made of silver atoms. Each atom keeps the unique properties of silver. Elements are pure substances made of identical atoms. They cannot be broken down into simpler substances by chemical changes. Most elements are atomic elements, which use single atoms as their basic unit. Carbon and sodium are examples of atomic...

molecular models

Molecular Model Types in Chemistry

Molecular models help chemists and students visualize the structures and shapes of atoms and molecules. They represent the molecular architecture of chemical compounds in a way that is easier to study than formulas alone. Skeletal models are simple two-dimensional drawings of compounds. They show the molecular framework, or the bonds, without drawing every atom. In this model, many carbon and hydrogen atoms are not shown directly. Their positions are implied by the ends and junctions of the...

organic compounds

Organic Molecules in Everyday Chemistry

Organic molecules are carbon-based compounds found in living things and in many everyday products. Most organic compounds have carbon as the main element, bonded to hydrogen and other carbon atoms. Some carbon-containing substances, such as carbonates, cyanides, and simple oxides like CO and CO2, are not classified as organic. Carbon can form up to four strong bonds, which helps explain why organic compounds can make chains and rings of many sizes and shapes. These molecules appear in...

formula mass and mole concepts of compounds

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

Chemical Formula Mass and Percent Composition

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance. For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses. Ionic...

experimental determination of chemical formula

Chemical formulas show the elemental makeup of a compound. When a formula is unknown, scientists often start by measuring the masses of the elements in the sample. Those mass data are then used to find the simplest whole-number ratio of atoms, which gives the empirical formula. To get from mass to atom ratio, the masses of the elements are first converted to moles using molar masses. Chemical formulas describe numbers of atoms, not masses, so this step is necessary. In one example, 1.71 grams...

chemical equations

Reading Chemical Equations and Balancing Them

Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products. The numbers in a chemical equation are shown with coefficients. A coefficient...

key terms and concepts

Learn how to use mass, percent composition, and molar mass to determine empirical formulas and molecular formulas from experimental data.

Finding Empirical and Molecular Formulas | JoVE K12

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

PS.Chm.4.3

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Structure And Properties Of Matter

Quantifying matter

Interactions Of Matter

Stoichiometry

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.a

CH.3.b

CH.3.c

Texas Essential Knowledge and Skills

High School

CHEM.7

CHEM.7.B

CHEM.8

CHEM.8.D

CHEM.9

CHEM.9.A

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.4.a

Florida Department of Education

Physical Science

SC.912.P.8

SC.912.P.8.7

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-7.

Georgia Standards of Excellence

SC2.

Predicting Chemical Formulas

Scientific Laws and Theories

Scientific Laws

In science, a law is defined as a concise, verbal or mathematical, statement that summarizes a vast number of experimental observations. ...

The Scientific Method

Chemistry is an empirical science. Scientists often pose questions to understand the chemistry in everyday life and seek answers to these questions. To ...

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

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 ...

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 ...

Use models to explain how the scientific understanding of atomic structure has evolved.

PS.Chm.1.1

Radioactivity and Nuclear Equations

Nuclear chemistry is the study of reactions that involve changes in nuclear structure. The nucleus of an atom is composed of protons and, except for ...

Types of Radioactivity

The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
Alpha (&#945;) decay is the ...

Nuclear Stability

Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10&#8722;15 meters, a nucleus is ...

Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

PS.Chm.1.2

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 Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

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 ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

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 ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

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, ...

Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

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 ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

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 ...

Hydrogen Bonds

Hydrogen BondsHydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, ...

Van der Waals Interactions

Atoms and molecules interact with each other through intermolecular forces. These electrostatic forces arise from attractive or repulsive interactions ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Finding Empirical and Molecular Formulas

Description

Transcript

More videos by Standards