1. Aim: How to calculate Percent Composition
DO NOW:
In pairs think about and write down the steps you think are necessary to solve the following problem.
What percent of the total mass of H2O is due to the mass of hydrogen?
How to Calculate % Composition

2. Percent Composition (Table T)
The percent composition is the mass percentage of each type of atom (element) in a compound.
% by mass = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑝𝑎𝑟𝑡 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒 x 100
How to Calculate % Composition

3. Percent Composition from Chemical Formula
To calculate the percent composition of a component in a compound:
Find the molar mass of the compound by adding up the masses of each atom in the compound.
Calculate the molar mass of the component in the compound for which you are solving.
Divide the molar mass of the component by the total molar mass of the compound and multiply by 100
How to Calculate % Composition

4. Percent Composition from Chemical Formula
Calculate the percent by mass of nitrogen in caffeine (C8H10N4O2)
Step1
8C + 10H +4N + 2O
(8 x 12.0 g/mol) + (10 x 1.0 g/mol) + ( 4 x g/mol) + (2 x 16.0 g/mol) = g/mol
How to Calculate % Composition

5. Nitrogen: 4 𝑥 14.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100 = 28.9 %
Step 2
Nitrogen: 4 𝑥 14.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100 = 28.9 %
How to Calculate % Composition

6. Percent Composition from Mass Data
Percent composition can be calculated from experimental data.
A student prepares a compound of tungsten chloride from g of tungsten and g of chlorine. Calculate the percent composition.
% W = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊 𝐶𝑙 2 x 100% = 𝑔 x 100 = 50.9 %
% Cl = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐶𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑊 𝐶𝑙 2 x 100% = 𝑔 x 100 = 49.1 %
How to Calculate % Composition

7. Percent Composition of Crystal Hydrates
A HYDRATE is a CRYSTALLINE compound in which ions are attached to one or more WATER molecules.
Example: Na2CO3 10H2O
 Notice how WATER molecules are BUILT INTO the chemical formula
 Substances without water built into the formula are called ANHYDRATES.
How to Calculate % Composition

8. Problem: What is the percentage by mass of water in sodium carbonate crystals (Na2CO3 10H2O)?
Step 1: Calculate the formula mass for the hydrate.
Na = 2 x 23.0 g/ mol = g/mol
C = 1 x g/mol = 12.0 g/mol
O = 3 x g/mol = 48.0 g/mol
H2O = 10 x 18.0 g/mol = 180 g/mol
Formula mass of hydrate = 286 g/mol
How to Calculate % Composition

9. Step 2: Set up and Calculate
% composition by mass = mass of part/ mass of whole X 100%
% H2O by mass =(180 g/mol)/(286 g/mol) x 100%
% H2O by mass = 62.9 %
How to Calculate % Composition

10. Practice Problems In pairs discuss and write down the steps
A 45.9 g sample of copper, Cu, combined with oxygen to give g of an oxide compound. The percent composition of this compound is:
88.8 % Cu, 11.2% O
24.4% Cu, 75.6% O
60.1% Cu, 39.9 % O
11.3% Cu, 88.7% O
How to Calculate % Composition