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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.
© 2014 Pearson Education, Inc.
Molar Mass
© 2014 Pearson Education, Inc.
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
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
© 2014 Pearson Education, Inc.
Molar Mass of a Compound
Calculate the molar mass of lithium carbonate,
Li2CO3, used to produce the red color infireworks.
© 2014 Pearson Education, Inc.
Learning Check
Calculate the molar mass of Li2CO3.
Step 1 Obtain the molar mass of each element.From the periodic table:
© 2014 Pearson Education, Inc.
Solution
© 2014 Pearson Education, Inc.
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
© 2014 Pearson Education, Inc.
Solution
© 2014 Pearson Education, Inc.
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).
Calculate the molar mass of potassium chloride,KCl.
Step 1 Obtain the molar mass of each element.
© 2014 Pearson Education, Inc.
Solution
© 2014 Pearson Education, Inc.
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