Molecular and Formula Masses

Using atomic masses from the periodic table and a molecular formula, we can determine the molecular mass, which is the mass in atomic mass units (amu) of an individual molecule. The molecular mass is simply the sum of the atomic masses of the atoms that make up the molecule. We multiply the atomic mass of each element by the number of atoms of that element in the molecule and then sum the masses for each element present. For example, 

Because the atomic masses on the periodic table are average atomic masses, the result of such a determination is an average molecular mass, sometimes referred to as the molecular weight. As with the term atomic mass, we will use the term molecular mass in this text. 

Although an ionic compound does not have a molecular mass, we can use its empirical formula to determine its formula mass (the mass of a formula unit ), sometimes called the formula weight. Sample Problem 3.1 Qlustrates how to determine molecular mass and formula mass. 

Think About It Double-check that you have counted the number of atoms correctly for each compound and that you have used the proper atomic masses from the periodic table. 

Calculate the molecular mass or the formula mass, as appropriate, for each of the following compounds (a) propane, CsHg, (b) lithium hydroxide, LiOH, and (c) barium acetate, Ba(C2H302)2. 

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Skill 8.3 Solving problems involving molecular and formula masses and percent composition... 

Determining molecular and formula masses of compounds is a skill you will use over and over as you master chemical calculations. Be certain you can do it with ease before moving on. 

It is all very well to calculate the atomic, molecular, and formula masses of atoms, molecules, and other compounds, but since we cannot weigh an individual particle, these masses have a limited usefulness. To make measurements of mass useful, we must express chemical quantities at the macroscopic level. The bridge between the particulate and the macroscopic levels is molar mass, the mass in grams of one mole of a substance. The units of molar mass follow from its definition grams per mole (g/mol). Mathematically, the defining equation of molar mass is... 

It may be said that because atomic, molecular, and formula masses are all based on carbon-12, they are conceptually alike. What then are their differences ... 

In what units are atomic, molecular, and formula mass expressed Define those units. 

Molecular and Formula Masses Percent Composition of Compounds Chemical Equations... 

How do we use molecular masses Because the mass ratio of one HC1 molecule to one ethylene molecule is 36.5 28.0, the mass ratio of any given number of HC1 molecules to the same number of ethylene molecules is always 36.5 28.0. In other words, a 36.5 28.0 mass ratio of HC1 and ethylene corresponds to a 1 1 number ratio. Equal numbers of different molecules (or formula units) always have a mass ratio equal to their molecular (or formula) mass ratio (Figure 3.1). 

When referring to the enormous numbers of molecules or ions that take part in a visible chemical reaction, it s convenient to use a special unit called a mole, abbreviated mol. One mole of any substance is the amount whose mass—its molar mass—is equal to the molecular or formula mass of the substance in grams. One mole of ethylene has a mass of 28.0 g, one mole of HC1 has a mass of 36.5 g, one mole of NaCl has a mass of 58.5 g, and so on. (To be more precise, one mole is formally defined as the amount of a substance that contains the same number of molecules or formula units as there are atoms in exactly 12 g of carbon-12.)... 

For work in the laboratory, it s necessary to weigh reactants rather than just know numbers of moles. Thus, it s necessary to convert between numbers of moles and numbers of grams by using molar mass as the conversion factor. The molar mass of any substance is the amount in grams numerically equal to the substance s molecular or formula mass. Carrying out chemical calculations using these relationships is called stoichiometry. 

In practice, one mole is the relative atomic or molecular or formula mass of any substance expressed in grammes, and it contains 6 X 10 particles. We do not have to go around counting the particles instead we can use the relative masses of elements or compounds. These are easily measured by weighing quantities. 

Although the terms molecular mass and formula mass are different, and they really do represent different concepts, the method for calculating each is identical. In fact, you might not even know if the particular compound that you are calculating the mass for is molecular or ionic, but you can still get the correct value. Don t get thrown off by the language in the questions that you are asked to answer. If the question asks for the molecular mass, that is simply an indication that you are working with a molecular compound. If the question refers to formula mass, it is simply noting that the compound is ionic. 

Find the molar mass of the solute, CaCl2. Remember The process of finding the molar mass of a substance is the same as finding the molecular or formula mass of the substance. We look up the masses listed on the Periodic Table of Elements for each of the elements involved and multiply by the appropriate subscripts. The only difference is that you use the unit symbol g for grams, instead of u for atomic mass units. 

Because it is convenient to have quick access to the atomic masses of the elements, the atomic mass of the 40 most often used elements appears in the following table. Of course, the atomic mass for any element can be found on any periodic table. The unit of atomic mass, molecular mass, and formula mass is the atomic mass unit, amu. In some texts, this unit is omitted when solving problems, while in other texts it is used consistently. Because It is important to get accustomed to using units, the amu is used in this text. 

Whether you are dealing with elements or compounds, the molar mass of a species is the mass in grams of 1 mole (6.022 x 1023) of that species 1 mole of atoms, 1 mole of molecules, or 1 mole of formula units. With compounds, you re dealing with molecules and formula units, so it is necessary to calculate the molecular or formula mass of each compound to get its molar mass. It s easy to get confused by the language ... 

FOLLOW-UP PROBLEM 2.13 What is the formula and molecular (or formula) mass of each of the following compounds (a) hydrogen peroxide (b) cesium chloride (c) sulfuric acid (d) potassium sulfate ... 

Chemical formulas describe the simplest atom ratio (empirical formula), actual atom number (molecular formula), and atom arrangement (structural formula) of one unit of a compound. An ionic compound is named with cation first and anion second. For metals that can form more than one ion, the charge is shown with a Roman numeral. Oxoanions have suffixes, and sometimes prefixes, attached to the element root name to indicate the number of oxygen atoms. Names of hydrates give the number of associated water molecules with a numerical prefix. Acid names are based on anion names. Covalent compounds have the first word of the name for the element that is leftmost or lower down in the periodic table, and prefixes show the number of each atom. The molecular (or formula) mass of a compound is the sum of the atomic masses in the formula. Molecules are depicted by various types of formulas and models. 

Each element in a compound constitutes its own particular portion of the compound s mass. For an individual molecule (or formula unit), we use the molecular (or formula) mass and chemical formula to find the mass percent of any element X in the compound ... 

Realize the usefulness of the mole concept, and use the relation between molecular (or formula) mass and molar mass to calculate the molar mass of any substance ( 3.1) (EPs 3.1-3.5, 3.7-3.10)... 

Therefore, 58.44 grams of sodium chloride will have the same number of fundamental units as 32.00 grams of O2, and 18.02 grams of H2 O whose formulas describe molecules. So, in the case of substances which exist as molecules, we can call the sum of atomic masses the molecular mass. But how shall we name the mass of sodium chloride equivalent to those masses. We know that the designation, molecule, for NaCl is not appropriate. However, if we describe the mass of the fundamental unit representing the composition of any compound by using the expression formula mass, such difficulties are avoided. Table 4-1 lists the formulas and formula masses of several representative compounds. 

The mass in grams of 1 mol of a compound is called the molar mass. The term molar mass applies to both covalent and ionic compounds. Keep in mind that we use the terms formula mass and molecular mass to refer to ionic and covalent compounds, respectively (Section 4.1). Molar mass is given in units of grams while formula mass and molecular mass are given in units of amu. (Note that the terms molar mass and formula mass apply to elements as well as compounds.)... 

One mole of a compound contains Avogadro s number of formula units of that compound. The terms molecular weight, molecular mass, formula weight, and formula mass have been used in the past to refer to the mass of 1 mol of a compound. However, the term molar mass is more inclusive, because it can be used for all types of compounds. 

Problem Each scene represents a binary compound. Determine its formula, name, and molecular (or formula) mass. 

Plan Each of the conqrounds contains only two elements, so to find the formula, we find the simplest whole-number ratio of one atom to the other. Erom the formula, we determine the name and the molecular (or formula) mass. 

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