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Determining Formula and Molar Masses Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2014 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: November 16, 2014 www.ck12.org C HAPTER Chapter 1. Determining Formula and Molar Masses 1 Determining Formula and Molar Masses Lesson Objectives The student will: • calculate the formula mass of a compound given its name and a periodic table. Vocabulary • formula mass • molecular mass Introduction When atoms of one element chemically combine with atoms of another element, a compound is formed. Compounds have names and formulas associated with them. The formula of a compound contains chemical symbols that tell us what elements are in the compound. The subscripts in the formula tell us the ratios of the elements present. For example, the formula MgCl2 tells us that this compound is composed of the elements magnesium and chlorine, which combine in the ratio of two atoms of chlorine for each atom of magnesium. Using the formula of the compound and the relative masses of elements from the periodic table, we can then calculate the relative formula mass for the compound. Formula Mass The periodic table tells us the relative masses of all the elements. Looking at the squares for carbon and helium, we can see that a carbon atom has about three times the mass of a helium atom. In this way, we can compare the relative masses of any two atoms in the table. By looking at the chemical formulas and the relative atomic masses, we can also compare the masses of different compounds. The formula mass of a compound is the sum of all the atomic masses in the formula. The formula for water, for example, is H2 O. This formula tells us that water is composed of hydrogen and oxygen and has a ratio of two hydrogen atoms for each oxygen atom. We can determine the formula mass for water by adding up the atomic masses of its components. Example: What is the formula mass of H2 O? Solution: Element Atomic Mass Number of Atoms per Formula H 1.0 daltons 2 O 16.0 daltons 1 Product 2.0 daltons 16.0 daltons 18.0 daltons The formula mass of H2 O = 18.0 daltons. Example: 1 www.ck12.org What is the formula mass of Ca(NO3 )2 ? Solution: Element Atomic Mass Number of Atoms per Formula Ca 40.0 daltons 1 N 14.0 daltons 2 O 16.0 daltons 6 Product 40.0 daltons 28.0 daltons 96.0 daltons 164.0 daltons The formula mass of Ca(NO3 )2 = 164.0 daltons. These formula masses are in the same units as atomic masses and therefore are exactly comparable. For example, the atomic mass of an oxygen atom is 16 daltons, the atomic mass of a fluorine atom is 19 daltons, and the formula mass of a water molecule is 18 daltons. This means that a water molecule is slightly more massive than an oxygen atom and slightly less massive than a fluorine atom. Terminology Since ionic compounds do not form individual molecules, the term formula mass is the only proper term to describe this relative mass. In comparison, the formula mass for covalent compounds may also be called the molecular mass because covalent compounds do form molecules. While it is important to understand this distinction, no professional chemists have had their degrees in chemistry recalled when they referred to the “molecular mass” of NaCl due to a slip of the tongue. Lesson Summary • The molecular mass of a molecule is found by adding the atomic masses of all the atoms in one molecule. • Not all substances exist as molecules, so the term molecular mass is not used for all substances. The masses of ionic compounds and empirical formulas are called formula mass. Review Questions 1. Calculate the formula mass for each of the following. a. b. c. d. e. f. g. h. K2 SO4 CuO Mg3 (AsO4 )2 Ca3 (PO4 )2 Fe2 O3 Al(OH)3 (NH4 )2 S C12 H22 O11 2. On average, how many times heavier are bromine atoms than neon atoms? 3. An unknown element, M, combines with oxygen to form a compound with a formula of MO2 . If 25.0 grams of the unknown element combines with 4.50 grams of oxygen, what is the atomic mass of M? Answers: a. 174 b. 79.5 2 www.ck12.org Chapter 1. Determining Formula and Molar Masses c. 351 d. 310 e. 165 f. 78 g. 68 h. 342 2. 4 times larger 3. 177.8 g/mol 3