1. Aim: How to calculate Percent Composition
DO NOW:
What is the number of moles of potassium chloride present in 148 g?
What is the molar mass of CuSO4 5H2O?
What is the mass of 3.01 x 1023 atoms of carbon?
How to Calculate % Composition

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

3. 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

4. Percent Composition from Chemical Formula
% mass = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 1 𝑚𝑜𝑙 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 x 100%
How to Calculate % Composition

5. 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

6. Percent Composition from Chemical Formula
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

7. Carbon: 8 𝑥 12.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100% = 49.5 %
Step 2
Carbon: 8 𝑥 12.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100% = 49.5 %
Hydrogen: 10 𝑥 1.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100% =5.1 %
Nitrogen: 4 𝑥 14.0 𝑔/𝑚𝑜𝑙 194 𝑔/𝑚𝑜𝑙 x 100% = 28.9 %
Oxygen: 100% - ( ) = 16.5 %
How to Calculate % Composition

8. 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

9. 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

10. 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

11. Percent Composition as a conversion Factor
We can use mass percent composition as a conversion factor between grams of an element and grams of the compound.
The mass percent composition of sodium in sodium chloride is 39%.
This can be written as:
39 g of Sodium : 100 g of Sodium Chloride
How to Calculate % Composition

12. This can be written in fractional form:
These fractions are conversion factors between grams of Na and grams of NaCl.
How to Calculate % Composition

13. The FDA recommends that adults consume less than 2
The FDA recommends that adults consume less than 2.4 g of sodium per day. How many grams of sodium chloride can you consume and still be within the FDA guidelines? Sodium chloride is 39% sodium by mass.
Known Unknown
2.4 g Na grams of NaCl
39 g Na : 100 g NaCl
2.4 g Na x 100 𝑔 𝑁𝑎𝐶𝑙 39 𝑔 𝑁𝑎 = 6.2 g NaCl
How to Calculate % Composition

14. Practice Problems
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

15. 2. Calculate the percent compost ion of each set: H2O H=___11
2. Calculate the percent compost ion of each set: H2O H=___11.1%_____ O=______88.9%______ H2O2 H = ____5.88%____ O =_______94.1%_____
How to Calculate % Composition

16. 3. How many grams of sodium will combine with 567
3. How many grams of sodium will combine with g of Sulfur from Na2S? (Hint: use % composition as conversion factor)
815.6 g
How to Calculate % Composition