1. Mass Percent
Chemistry 11

2. Law of Definite Proportions
The elements in a given chemical compound are always present in the same proportions by mass
The mass of an element in a compound, expressed as a percent of the total mass of the compound, is the element’s mass percent

3. Magnesium Oxide
We completed a lab involving the combustion of magnesium to create magnesium oxide
From theory, we know that the formula for magnesium oxide should be MgO
One of the groups discovered that the mass percents were as follows:
Mg – 58%
O – 42%

4. Magnesium Oxide What should the mass percent be? Molar Masses
Magnesium – 24.31g/mol
Oxygen – 16.00g/mol
Magnesium oxide – 40.31g/mol

5. Different Compounds – Same Elements
Determine the mass percent of the following two hydrogen/oxygen compounds:
Water
Hydrogen peroxide
Determine the mass percent of the following two carbon/oxygen compounds:
Carbon dioxide
Carbon monoxide

6. Different Compounds – Same Elements
Determine the mass percent of the following two hydrogen/oxygen compounds:
Water
H – 11.2%
O – 88.8%
Hydrogen peroxide
H – 5.94%
O – 94.1%
Determine the mass percent of the following two carbon/oxygen compounds:
Carbon dioxide
C – 27.3%
O – 72.7%
Carbon monoxide
C – 42.9%
O – 57.1%

7. Percent Composition From Mass Data
A sample of a compound has a mass of 48.72g. The sample is found to contain 32.69g of zinc and 16.03g of sulphur. What is the percentage composition of thie compound?

8. Practice Problems
Page 82
Questions 1-4