1. Find the % by mass of oxygen in water
Two things you need to know
Mass of part
Mass of whole
H2O
2 atoms of hydrogen
1 atom of oxygen
2 x 1.0 g/mol
1 x 16.0 g/mol
16.0 g/mol oxygen
2.0 g/mol hydrogen
2.0 g/mol g/mol = 18.0 g/mol for H2O
16.0 g/mol
88.9% oxygen
x 100% =
18.0 g/mol
Episode 702

2. NaHCO3 + HCl  NaCl + CO2 + H2O
Lab Excerise
We are going to react sodium bicarbonate with hydrochloric acid
NaHCO3 + HCl  NaCl + CO2 + H2O
Measure and determine the mass of CO2 produced in the reaction
Determine the % carbon in the CO2 produced
Determine the % carbon in the NaHCO3 used
Episode 702

3. Find the % carbon in sodium bicarbonate (NaHCO3)
Remember:
(mass of element in 1 mol of compound ÷ molar mass of compound) x 100%
Na 1 x 23 g/mol = 23 g/mol
H 1 x 1 g/mol = 1 g/mol
C 1 x 12 g/mol = 12 g/mol
O 3 x 16 g/mol = 48 g/mol
84 g/mol NaHCO3
12 g/mol
14.3% carbon
x 100% =
84 g/mol
Episode 702

4. Find the % composition of aluminum oxide
Write formula
Al2O3
Calculate molar mass
Al 2 x 27 g/mol = 54 g/mol
O 3 x 16 g/mol = 48 g/mol
102 g/mol
54 g/mol
x 100% =
52.9% aluminum
102 g/mol
48 g/mol
47.1% oxygen
x 100% =
102 g/mol
Episode 702

5. Empirical Formula
Simplest whole number ratio of elements in a compound
Example
Find the empirical formula for a compound containing 56.6g K, 8.7g C, and 34.7g O
Step 1: Convert each mass into moles of the element.
56.6g K
1mol K
K1.45C0.73O2.17
= 1.45 mol K
39.1g K
Step 2: Divide by the smallest to find a simple whole number.
8.7g C
1mol C
= 0.73 mol C
12g C
K2CO3
34.7gO
1mol O
= 2.17 mol O
16gO
Episode 702

6. Na2SO4 Na1.41S0.705O2.81 32.37g Na 22.64g S 44.99g O 100 g 1.41 mol Na
100 %
(Hint: When % are given, assume you have 100g of the compound, and the % changes to grams.)
32.37g Na
22.64g S
44.99g O
100 g
1.41 mol Na
0.705 mol S
2.81 mol O
Step 1: Convert each mass into moles of the element.
Na1.41S0.705O2.81
Step 2: Divide by the smallest to find a simple whole number.
Na2SO4
Episode 702

7. PxOy
10.15 g sample
4.433 g P
Determine the grams of oxygen in the sample.
10.15g – 4.433g = 5.717g oxygen
Step 1: Convert each mass into moles of the element.
4.433g P
1 mol P
5.717g O
1mol O
= mol P
= mol O
31g P
16g O
P0.143O0.357
Step 2: Divide by the smallest to find a simple whole number.
PO2.5
(Hint: If the ratio is still not whole numbers, multiply both subscripts by a number, such as “2” to get rid of fraction.)
P2O5
Episode 702