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%