Everything Science Grade 10

CHAPTER 19. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 19.2

4. Problems where you will be asked tofindnumber of moles ofwaters of crystallisa-

tion.

The following worked examples will show you how to do each of these.

See video: VPbue at http://www.everythingscience.co.za

Example 7: Calculating the percentage by mass of ele-

ments in a compound

QUESTION

Calculate the percentage that each element contributes to the overall mass of sul-

phuric acid (H 2 SO 4 ).

SOLUTION

Step 1:Calculate the molar masses

Hydrogen= 2× 1 ,01 = 2, 02 g·mol−^1

Sulphur= 32, 1 g·mol−^1

Oxygen= 4× 16 ,0 = 64, 0 g·mol−^1

Step 2: Use the calculations in the previous step to calculate the molecular

mass of sulphuric acid.

Mass= 2, 02 g·mol−^1 +32, 1 g·mol−^1 +64, 0 g·mol−^1 = 98, 12 g·mol−^1

Step 3:Use the equation

Percentage by mass=molecular mass of Hatomic mass

2 SO 4

×100%

Hydrogen

2 , 02 g·mol−^1

98 , 12 g·mol−^1

×100% = 2,0587%

Sulphur

32 , 1 g·mol−^1

98 , 12 g·mol−^1

×100% = 32,7150%

Oxygen

64 , 0 g·mol−^1

98 , 12 g·mol−^1

×100% = 65,2263%

Chemistry: Chemical change 343