1. 3.8 -3.9 Formula Mass and Composition of Compounds

2. Molar Mass Equivalent to formula mass, except with the units g/mol.
What is the molar mass of sodium bicarbonate?
NaHCO3
Na = g/mol
H = 1.01 g/ mol
C = g/mol
O = g/mol X 3 = g/mol
NaHCO3 = g/mol g/mol g/mol g/mol = g/mol

3. 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 = amu
H2O = amu amu = amu

4. 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 = %
61.98 g/mol

5. Using Molar Mass to Count Molecules by Weighing
Find the number of ibuprofen molecules in a tablet containing mg of ibuprofen (C13H18O2)
Step 1: Convert mg  g mg = g
Step 2: Convert g  mol
0.200 g C13H18O g C13H18O2 =
X mol C13H18O2
X = 9.69X10-4 mol C13H18O2
Step 3: Covert mol  molecules
9.69X10-4 mol C13H18O mol C13H18O2 =
X X1023 molecules C13H18O2
X = 5.84X1020 molecules C13H18O2

6. 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 g Fe =
g Fe2O X
X = g Fe2O3

7. Conversion Factors from Chemical Formulas
Determine the mass of oxygen in 7.2 g sample of Al2(SO4) g/mol O % mass oxygen = X g/mol Al2(SO4)3 % mass oxygen = % g O X = g Al2(SO4)3 7.2 g Al2(SO4)3 X = 4.04 g oxygen

8. Pg. 132 (Just complete the a’s) #’s 60, 62, 64, 66, 68, and question 78 Read 3.10 pgs. 114 -119