Chapter 7 Lecture

Basic ChemistryFourth Edition

Chapter 7 Chemical Quantities7.2 Molar Mass

Learning Goal Given the chemical formula of a substance, calculate its molar mass.

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Molar Mass

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The molar mass is

• the quantity in grams that equals the atomic mass of that element

• the mass of one mole of an element or compound

• the mass of 6.022 × 1023 atoms of an element

• the mass of 6.022 × 1023 molecules or formula units

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Molar Mass of Carbon

To determine the molar mass of a compound,

• multiply the molar mass of each element by its subscript in the formula

• add the results and round to 0.01 gram

Molar mass NH3

1 mol NH3 1 mol N =1(14.01 g N) = 14.01 g N

3 mol H =3(1.01 g H) = 3.03 g H

Molar mass NH3 = 17.04 g NH3

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Molar Mass of a Compound

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Guide to Calculating Molar Mass

Calculate the molar mass of lithium carbonate,

Li2CO3, used to produce the red color infireworks.

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Learning Check

Calculate the molar mass of Li2CO3.

Step 1 Obtain the molar mass of each element.From the periodic table:

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Solution

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Solution

Calculate the molar mass of Li2CO3.

Step 2 Multiply each molar mass by the moles (subscript) in the formula.

2 mol Li × = 13.88 g Li

1 mol C × = 12.01 g C

3 mol O × = 48.00 g O

Calculate the molar mass of Li2CO3.

Step 3 Calculate the molar mass by adding the masses of the elements.

2 mol Li = 13.88 g Li1 mol C = 12.01 g C3 mol O = 48.00 g O

Molar mass Li2CO3 = 73.89 g Li2CO3

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Solution

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One-Mol Quantities

One-mol samples: sulfur, S (32.07 g); iron, Fe (55.85 g); salt, NaCl (58.44 g); potassium dichromate, K2Cr2O7 (294.2 g); and sucrose, C12H22O11 (342.3 g).

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Molar Mass of Elements, Compounds

Calculate the molar mass of potassium chloride,KCl.

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Learning Check

Calculate the molar mass of potassium chloride,KCl.

Step 1 Obtain the molar mass of each element.

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Solution

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Solution

Calculate the molar mass of potassium chloride, KCl.

Step 2 Multiply each molar mass by the moles (subscript) in the formula.

1 mol K x = 39.10 g K

1 mol Cl x = 35.45 g C

Calculate the molar mass of potassium chloride, KCl.

Step 3 Calculate the molar mass by adding the masses of the elements.

1 mol K = 39.10 g K1 mol Cl = 35.45 g Cl

Molar mass KCl = 74.55 g KCl

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Solution