EXAMPLE 1: Calculate the relative molecular mass of KOH. (K=39, O =16, H = 1)

**Solution**

The relative molecule mass = Sum of all the relative atomic masses of the atoms of element that make up the compound.

KOH = 1 x 39 + 1 x 16 + 1 x 1

= 39 + 16 + 1 = 56

Example 1: Calculate the molar mass of KOH

(K=39, O=16, H=1)

**Solution**

K = 39, O 16, H=1

KOH = 1 x 39 + 1 x 16 + 1 x 1

= 39 + 16 + 1 = 56g/mol

Example 2: How many moles are there in 20g of KOH

**Solution**

**Mole** = (reacting mass)/(Molar mass)

Here, reacting mass = 20g, molar mass = 56g/m

∴ Mole = 20/56 = 5/14 mole

**Percentage By Mass**

% By mass of an element = (Mass of the element)/(Total Molar Mass of compound ) x 100

Example 3: Calculate the percentage by mass of calcium in CaCO3

(Ca = 40, C = 12,O = 16)

**Solution**

CaCO3 = 40 + 12 +16 x 3

= 40 + 12 + 48

= 100

% by mass of calcium = (40/100)x100

% by mass of Calcium = 40%