Molar Mass and Percent Composition Practice WS (1)

Name: ______________________________________________________________Period:__________________Date:___________________________
Percent Composition Notes and Practice
Percent composition is the percent by mass of each element found in a compound.
Example: What percent of iron (III) hydroxide, Fe(OH)3, is oxygen?
Step 1: Find the molar mass of the compound.
1 mol Fe
= 1 x 55.85 g/mol =
3 mol O
= 3 × 16.00 g/mol =
3 mol H
= 3 × 1.008 g/mol =
Molar Mass Fe(OH)3
55.85 g/mol
48.00 g/mol
3.024 g/mol
106.87 g/mol
Step 2: To find the percent by mass, divide the part by the whole and multiplying by 100.
48.00 g O
× 100 %
106.87 g Fe(OH) 3
44.91% oxygen
Example: If you are given a 16.0 g sample of Fe(OH)3, how many grams of oxygen can be recovered?
Step 1: Determine the percent of oxygen in the compound.
This was already done above.
The percent of oxygen in this compound is 44.91%
Step 2: Multiply the starting mass by the percent of the desired element.
Report your answer to the correct number of significant figures.
16.0 g Fe(OH)3 x 0.4491 = 7.1856 g O  rounded to 3 SF  7.19 g O (final answer)
Solve the following.
1. What percent of glucose, C6H12O6, is carbon?
2. What percent composition of phosphorus in Zn3(PO4)2? This means to find the percent composition of each
element in the compound (Zn, P, and O)
3. What percent of FeSO4 • 6H2O is Fe?
4. What percent of AgNO3 is silver?
5. How many grams of aluminum can be recovered from a 34.5 g sample of Al2O3?
6. What is the molar mass of P4O10? This just means to add up the masses of all the atoms in the formula. See the
example on the front page.
7. Write the name of Fe2(CO3)3. _____________________________________________
How many grams of iron can be recovered from a 115 g sample of Fe2(CO3)3?
8. Write the name of SO3. ________________________________________________
In a 63.17 g sample of SO3, how many grams are sulfur?
9. What is the name of Cu(C2H3O2)2? _____________________________________________________
What is the percent of hydrogen in Cu(C2H3O2)2?