Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 56 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

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

Formula Mass and Mole Concepts of Compounds

化合物的最小代表单位 是分子（或分子式单位）用化学式描述。化学式中所有原子的原子质量，如果加在一起，可求出化合物的 分子质量（或分子式质量）以原子质量单位（amu）为单位。例如，为了求出蔗糖或食用糖的分子质量，用每个元素的原子质量 乘以相应元素的 原子数。总和就是蔗糖的分子质量。一摩尔化合物中所有分子 或分子式单位）的质量 称为它的摩尔质量，以克每摩尔为单位。化合物的摩尔质量在数值上 等于它的分子质量（或分子式质量）例如，分子质量为 342.3 原子质量单位的蔗糖，其摩尔质量 为 342.3 克每摩尔。了解化学物质中存在的分子或粒子 的实际数量是 理解其组成、相互作用和组合 的基础。然而，在给定的化合物样品中，用普通方法计算分子的实际数量 是不切实际的。通过利用摩尔质量、摩尔数 和阿伏伽德罗数之间的关系，可以计算出 存在的总分子数。根据概念，化合物的摩尔质量 相当于一摩尔的化合物，由 6.022×10^23 个粒子组成。因此，342.3 克的蔗糖含有 6.022×10^23 个蔗糖分子。然而，比如说如果一个蔗糖样品的重量为 150 克，那么分子的数量可以通过 使用交叉乘法的方法计算确定。结果表明，150 克的蔗糖构成 2.63 10^23 个蔗糖分子。化合物中每种元素的百分比 可以用质量百分比值来表示。计算时，元素在 一摩尔化合物中的质量除以 一摩尔化合物的总质量，然后转换成百分比。实际上，要计算蔗糖中氢的 质量百分比，氢原子的总质量 和蔗糖分子的总质量 都包含在分子式中。计算结果表明蔗糖含有 6.4 质量百分比的氢。

化合物的分子质量和摩尔概念

<h4>共价化合物的分子质量</ h4>
 
 化学式表示物质的元素组成。对于共价化合物，分子式表示组成该物质单个分子的原子的数量和类型；因此，式质量可以正确地称为分子质量。氯仿（CHCl 3 ）的共价化合物的分子式表明，一个分子包含一个碳原子，一个氢原子和三个氯原子。氯仿分子的平均分子量等于这些原子的平均原子质量之和：
 
 <img alt =“ Eq1” src =“ https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png” style =“ height: 40px;“ /> 
 
 <h4>离子化合物的分子质量</ h4>
 
 离子化合物由离散的阳离子和阴离子组成，这些阳离子和阴离子按比例组合以产生电中性的块状物质。离子化合物的分子质量的计算方法与共价化合物的分子质量的计算方法相同：将化合物分子式中所有原子的平均原子质量相加。但是，离子化合物的分子式不能代表离散分子的组成，因此不能正确地称为“分子质量”。
 
 例如，普通食盐或氯化钠（NaCl）是由钠阳离子（Na + ）和氯离子（Cl ＆minus; ）组合而成的离子化合物以1：1的比例。该化合物的分子式是通过将其组成元素的平均原子质量相加得出的：
 
 <img alt =“ Eq2” src =“ https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png” style =“ height: 40px;“ /> 
 
 当计算离子化合物的分子式质量时，使用的是中性钠和氯原子的平均质量，而不是钠阳离子和氯离子的质量。即使钠阳离子的质量比钠原子小一些（因为它缺少电子），但氯离子比氯原子的质量略大（由于多余的电子），这一差异将被抵消。而且，电子的质量相对于典型原子的质量可以忽略不计。
 
 <h4>质量百分比组成</ h4>
 
 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。化合物中每种元素的质量百分比称为该特定元素的质量百分比。组成百分比可以通过将每种元素的质量除以化合物的总质量，然后转换为百分比来计算。
 
 百分比组成可用于评估已知化学式的不同化合物中给定元素的相对丰度。只要知道该化合物的分子式或经验式，就可以从该化合物元素的原子质量或摩尔质量中得出组成百分比。
 
 例如，一分子硝酸（HNO 3 ）包含一个氮原子，重14.01 amu；一个氢原子，重1.008 amu；三个氧原子，重（3×16.00）。 amu）= 48.00 amu。因此，硝酸的公式质量为（14.01 amu + 1.008 amu + 48.00 amu）= 63.02 amu，其百分组成为：
 
 <img alt =“ Eq3” src =“ https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-03.png” style =“ height: 40px;“ /> 
 
 <img alt =“ Eq4” src =“ https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-04.png” style =“ height: 40px;“ /> 
 
 <img alt =“ Eq5” src =“ https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-05.png” style =“ height: 40px;“ /> 
 
 此文本改编自<a href="https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept"> Openstax ，化学2e，第3.1节：分子式和摩尔概念</a>和<a href =“ https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular -formulas“> Openstax，化学2e，第3.2节：确定经验式和分子式。</a>

共价化合物的分子量

化学式表示物质的元素组成。对于共价化合物，其化学式表示……

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

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 compounds are different because they contain cations and anions that combine in ratios that make the bulk material electrically neutral. Their formula mass is found the same way, by adding the average atomic masses of the atoms in the formula. Sodium chloride, or table salt, is one example. Its formula, NaCl, shows sodium and chloride in a 1:1 ratio. For ionic compounds, the term molecular mass is not the best choice because the formula does not describe a single molecule.

When calculating formula mass for an ionic compound, neutral atom masses are used instead of ion masses. A sodium cation has slightly less mass than a sodium atom because it has lost an electron. A chloride anion has slightly more mass than a chlorine atom because it has gained an electron. The difference is small, and the mass of an electron is negligibly small compared with the mass of a typical atom.

Percent composition tells how much of a compound’s mass comes from each element. It is found by dividing the mass of each element by the total mass of the compound and converting the result to a percentage. This is useful for comparing the relative amount of an element in compounds with known formulas.

Nitric acid, HNO3, shows how the calculation works. One molecule contains one nitrogen atom with a mass of 14.01 amu, one hydrogen atom with a mass of 1.008 amu, and three oxygen atoms with a total mass of 48.00 amu. Its formula mass is 63.02 amu. From that total, the percent composition of each element can be determined.

Chemical Formula Mass and Percent Composition

chemical-formula-mass-and-percent-composition

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

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

experimental-determination-of-chemical-formula

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...

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 calculate formula mass for covalent and ionic compounds, then use it to find percent composition with examples like chloroform, salt, and nitric acid.

Chemical Formula Mass and Percent Composition | 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

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Ohio’s Learning Standards and Model Curriculum

Structure And Properties Of Matter

Quantifying matter

Interactions Of Matter

Stoichiometry

Tennessee Academic Standards

High School

Matter and Its Interactions

CHEM1.PS1.1

CHEM1.PS1.3

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.a

CH.4

CH.4.a

Arizona Science Standards

Physical Science

HS+C.P1

HS+C.P1U1.7

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New Jersey Student Learning Standards for Science

HS-PS1-7

Texas Essential Knowledge and Skills

CHEM.7

CHEM.7.B

CHEM.8

CHEM.8.A

CHEM.8.B

CHEM.9

CHEM.9.A

Alabama Course of Study Science

9-12.C.1.4.a

Florida Department of Education

SC.912.P.8

SC.912.P.8.7

SC.912.P.8.9

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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-7.

Georgia Standards of Excellence

SC3.

The Mole Concept and Avogadro’s Number

Scientific Laws and Theories

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Elements: Chemical Symbols and Isotopes

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Nuclear Power

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Atomic Radii and Effective Nuclear Charge

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Formal Charges

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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

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Intermolecular vs Intramolecular Forces

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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.

Chemical Formula Mass and Percent Composition

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