chapter 7 – the mole and chemical composition sec 2 - relative atomic mass and chemical formulas...

Chapter 7 – The Mole and Chemical Composition

Sec 2 - Relative Atomic Mass and Chemical Formulas

Average Atomic Mass and the Periodic Table

You have learned that you can use atomic masses on the periodic table tofind the molar mass of elements. Many of these values on the periodictable are close to whole numbers. However, most atomic masses are writtento at least three places past the decimal.

Most Elements Are Mixtures of Isotopes

You remember that isotopes are atoms that have different numbers ofneutrons than other atoms of the same element do. So, isotopes have differentatomic masses. The periodic table reports a average atomic massweighted average of the atomic mass of an element’s isotopes. Aweighted average takes into account the relative importance of eachnumber in the average. Thus, if there is more of one isotope in a typicalsample, it affects the



(mass of atom1 x % quantity1) + (mass of atom2 x % quantity2) +……..Average Atomic Mass = 100

Remember from Chapter 3 the formula to calculate the average atomic mass

Calculate the average atomic mass of Copper using the information below.• atomic mass of a Cu-63 atom = 62.94 amu• abundance of Cu-63 = 69.17%• atomic mass of Cu-65 = 64.93 amu• abundance of Cu-65 = 30.83%



(62.94 x 69.17) + (64.93 x 30.83)Mass of Copper =

100

63.55 amu



Chemical Formulas and Moles

Until now, when you needed to perform molar conversions, you weregiven the molar mass of compounds in a sample. Where does this molarmass of compounds come from? You can determine the molar mass ofcompounds the same way that you find the molar mass of individual elementsby using the periodic table.

Formulas Express Composition

You have learned that covalent compounds, such as water and hexachloroethane,consist of molecules as units. Formulas for covalent compounds show both the elements and the number of atoms of each element in a molecule. Hexachloroethane has the formula C2Cl6. Each molecule has 8 atoms covalently bonded to each other. Ionic compounds aren’t found as molecules, so their formulas do not show numbers of atoms. Instead, the formula shows the simplest ratio of cations and anions.



Formulas Are Used to Calculate Molar Masses

A formula tells you what atoms (or ions) are present in an element orcompound. So, from a formula you can find the mass of a mole of the substance,or its molar mass. To do this you use the formula to tell you how many of whichAtoms an to look up there mass from the periodic table.

Find the molar mass for each of the following compounds:

a. CsI b. C12H22O11 c. HC2H3O2

d. CaHPO4 e. I2 f. Mg3(PO4)2

259.8 g/mol

342.0 g/mol

60.0 g/mol

136.1 g/mol

253.8 g/mol

262.9 g/mol

chapter 7 – the mole and chemical composition sec 2 - relative atomic mass and chemical formulas...

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relative atomic mass

atomic mass of cu

molar mass of compounds

average atomic mass

molar mass of elements

chemical compositionsec

molar mass ofcompounds

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