3.8 -3.9 Formula Mass and Composition of Compounds
Molar Mass
• Equivalent to formula mass, except with the units g/mol.
What is the molar mass of sodium bicarbonate?
NaHCO3
Na = 22.99 g/mol
H = 1.01 g/ mol
C = 12.01 g/mol
O = 16.00 g/mol X 3 = 48.00 g/mol
NaHCO3 = 22.99 g/mol + 1.01 g/mol + 12.01 g/mol + 48.00 g/mol = 84.01 g/mol
Formula Mass
• Formula mass (a.k.a. molecular mass or molecular
weight) is the sum of the atomic masses of all the
atoms that make up a compound. Unit is amu.
What is the formula mass of H2O?
H = 1.01 amu X 2 = 2.02 amu
O = 16.00 amu
H2O = 16.00 amu +2.02 amu = 18.02 amu
Composition of Compounds
• Mass percent composition is used to calculate the
percentage of an element in a compound .
mass of the element in 1 mol of compound
% mass of an element = ------------------------------------------------------------ X 100
mass of 1 mol of compound
Calculate the mass percent of sodium in sodium oxide.
Formula is Na2O
45.98 g / mol
% mass of Na = ------------------- X 100 = 74.19 %
61.98 g/mol
Using Molar Mass to Count Molecules by Weighing
Find the number of ibuprofen molecules in a tablet containing 200.0 mg of ibuprofen
(C13H18O2)
Step 1: Convert mg  g 200.0 mg = 0.200 g
Step 2: Convert g  mol
0.200 g C13H18O2
206.31 g C13H18O2
------------------------ = ---------------------------X
1.00 mol C13H18O2
X = 9.69X10-4 mol C13H18O2
Step 3: Covert mol  molecules
9.69X10-4 mol C13H18O2
1.00 mol C13H18O2
-------------------------------- = ---------------------------------------------X
6.022X1023 molecules C13H18O2
X = 5.84X1020 molecules C13H18O2
Mass Percent Composition as a
Conversion Factor
What mass (in grams) of iron(III) oxide contains 58.7 g
of iron? Iron(III) oxide is 69.94% iron by mass.
69.94 g Fe
58.7 g Fe
------------------------------- = ------------------100.00 g Fe2O3
X
X = 83.93 g Fe2O3
Conversion Factors from Chemical
Formulas
Determine the mass of oxygen in 7.2 g sample of Al2(SO4)3
192.00 g/mol O
% mass oxygen = -------------------------------- X 100
342.14 g/mol Al2(SO4)3
% mass oxygen = 56.12 %
56.12 g O
X
--------------------------- = --------------------100.00 g Al2(SO4)3
7.2 g Al2(SO4)3
X = 4.04 g oxygen
Pg. 132 (Just complete the a’s) #’s 60, 62, 64, 66, 68, and
question 78
Read 3.10 pgs. 114 -119