1. Percentage Composition (page 118)

2.  The Law of Definite Composition deals with COMPOUNDS. This law states that the composition of any pure compound is constant. The elements that make up a compound are always present in the same ratio by mass.

3.  Water is always 11.21% hydrogen by mass and 88.79% oxygen by mass.  Sodium chloride is always 39.34% sodium by mass and 60.66% chlorine by mass.  This is true regardless of the size of the sample or where the sample comes from.

4.  To calculate the percent composition (percentage composition) of a compound  Calculate the molecular mass (molecular weight, formula mass, formula weight), MM, of the compound molecular massmolecular mass  Calculate the total mass of each element present in the formula of the compound  Calculate the percent compositon =  (total mass of element present ÷ molecular mass) x 100

5. In general the percent of element A in compound AB can be represented by %A = mass of A X 100% %A = mass of A X 100% mass of AB mass of AB

6. Determine the percentage composition of NaCl.

7. What is the percentage composition of Na 2 SO 4 ?

8. Assignment  Complete the handout given  Complete question 7 on page 119