1. Chem Extra #3: % Composition and The Formulas
Science 10H
Chem Extra #3:
% Composition and The Formulas

2. Objectives By the end of the lesson you should be able to;
Calculate the percent composition of an reactant and product
Determine the empirical and molecular formula
Objectives

3. Percent composition is the percent by mass of a component in a chemical formula
i.e. the percent composition of H2SO4 or the percent composition of water in CuSO4∙5H2O
Percent Composition

4. Practice! What is the percent composition of H2SO4?
Always assume there is 1 mol of the compound.
Determine the molar mass (98.1 g)
Find the total mass of each element:
Mass of H: 2 x 1.0 g = 2.0 g
Mass of S: 1 X 32.1g = 32.1 g
Mass of O: 4 X 16 g = 64.0 g
4. Determine the % of each element:
% of H: 2.0 g X 100% = 2.0%
98.1 g
% of S: 32.1 g X 100% = 32.7%
% of O:64.0 g X 100% = 65.2%
Worksheet time!
Practice!

5. The empirical formula is the simplest formula and is the smallest whole-number ratio of the atoms which represent its composition
EX: CH2, C2H4, C3H6, C4H8 and C5H10 all contain twice as many H’s as C’s and all have the same empirical formula
CH2
Finding the empirical formula is the opposite of finding the percent composition
Empirical Formulas

6. Examples Remember that CH3 is the SIMPLEST formula!
What is the empirical formula of a compound consisting of 80.0% C and 20% H?
If you are not given molar mass or the formula assume you have 100 g of that compound
- mass of C: 80.0 % of 100 g = 80.0 g
- mass of H: 20.0% of 100 g = 20.0 g
2. Calculate the # of moles of each
- moles of C: 80.0 g X 1 mol = 6.67 mol
12.0 g
- moles of H: 20.0 g X 1 mol = 20 mol
1.0 g
Mol Ratio
÷ 6.67 mol 1
So….CH3 3
Examples
Remember that CH3 is the SIMPLEST formula!

7. 2. A compound contains 58. 5% C, 7. 3% H and 34. 1% N
2. A compound contains 58.5% C, 7.3% H and 34.1% N. What is the empirical formula of the compound? 3. What is the empirical formula of a compound containing 81.8% C and 18.2% H? 4. What is the empirical formula of a compound containing 39.0 % Si and 61.0% O?
Practice

8. Decimals to Fractions Equivalents
0.20 = 1/ = 1/ = 1/ = 2/ = 1/ = 3/ = 2/ = 3/ = 4/5 ALWAYS CARRY OUT EQUATIONS TO 4 DIGITS IN ORDER TO ACCURATELY CLEAR FRACTIONS
Decimals to Fractions Equivalents

9. The molecular formula a multiple of the empirical formula
RECALL: CH2, C2H4, C3H6, C4H8 and C5H10 all contain twice as many H’s as C’s and all have the same empirical formula…CH2
SO, we can use the ratio of the molar mass of the empirical formula to the molar mass of the molecular formula to figure out which multiple (N) we are dealing with
N = molar mass
empirical mass
Molecular Formula

10. Before we can jump into calculating the ratio we need to calculate the molar mass (g/mol) first. There are 4 possible ways depending on what information you are given:
Density of a gas at STP: use conversion factor 22.4L/mol
Mass and vol of a gas at STP: use m/v to get density then use the same conversion factor as #1
# moles and mass: just divide g/mol!
As a multiple of X: get the molar mass of X then multiply!
Finding Molar Mass

11. When you have the molar mass you can calculate the ratio!
N = molar mass
empirical mass
Once you have N all you need to do is multiply it through the empirical formula!
Finding Molar Mass

12. A molecule has an empirical formula of HO and a molar mass of 34. 0g
A molecule has an empirical formula of HO and a molar mass of 34.0g. What is the molecular formula?
A gas has the empirical formula POF3. If L of the gas at STP has a mass of 1.62 g, what is the molecular formula of the compound?
The empirical formula of a compound is SiH3. If mol of a compound has a mass of 1.71 g, what is the compound’s molecular formula?
Worksheet time!
Practice!