Everything Science Grade 11

6.4 CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE

QUESTION

Calculate the molar mass of H 2 SO 4.

SOLUTION

Step 1 : Use the periodic tableto find the molar mass for each element in the molecule.

Hydrogen = 1. 008 g·mol−^1 ; Sulphur = 32, 07 g·mol−^1 ; Oxygen = 16 g·mol−^1

Step 2 : Add the molar massesof each atom in the molecule

M(H 2 SO 4 )= (2× 1 .008) + (32.07) + (4× 16) = 98. 09 g· mol−^1

Example 7: Calculating moles frommass

QUESTION

Calculate the number of moles there are in 1 kgof MgCl 2.

SOLUTION

Step 1 : Write the equation forcalculating the numberof moles in the sample.

n =

m

M

Step 2 : Calculate the values that you will need, to substitute into the equation

1. Convert mass into grams
   m = 1kg× 1000 = 1000g


2. Calculate the molar mass of MgCl 2.

M(MgCl 2 )= 24.31 + (2× 35 .45) = 95. 21 g· mol−^1

Step 3 : Substitute values into the equation

n =

1000

95. 21

= 10. 5 mol

There are 10.5 moles ofmagnesium chloride ina 1 kg sample.