1.28: Understand How to Carry Out Calculations Involving Amount of Substance, Relative Atomic Mass (Ar) and Relative Formula Mass (Mr)

MOLE CALCULATIONS

          

1. CALCULATING MOLES

s

EQUATION:

s

Amount in Moles     =     Mass of Substance in Grams    ÷      Mr

s

Example:

s

SUBSTANCE	MASS	Mr	AMOUNT
NaOH	80 g	40	( 80 ÷ 40 ) = 2 Mol
CaCO3	25 g	100	( 25 ÷ 100 ) = 0.25 Mol
H2SO4	4.9 g	98	( 4.9 ÷ 98 ) = 0.05 Mol
H2O	108 g	18	( 108 ÷ 18 ) = 6 Mol
CuSO4.5H2O	75 g	250	( 75 ÷ 250 ) = 0.3 Mol

2. CALCULATING MASS

s

EQUATION:

s

Mass of Substance (Grams)     =     Moles    x      Mr

s

Example:

s

SUBSTANCE	AMOUNT	Mr	MASS
H2O	0.5 Mol	18	( 0.5 x 18 ) = 9 g
NaCl	3 Mol	58.5	( 3 x 58.5 ) = 175.5 g
K2CO3	0.2 Mol	138	( 0.2 x 138 ) = 27.6 g
(NH4)2SO4	2.5 Mol	132	( 2.5 x 132 ) = 330 g
MgSO4.7H2O	0.25 Mol	246	( 0.25 x 246 ) = 61.5 g

3. CALCULATING RELATIVE FORMULA MASS

EQUATION:

s

Mr     =     Mass of Substance in Grams     ÷     Moles

s

Example:

s

10 Mol of Carbon Dioxide has a Mass of 440 g. What is the Relative Formula Mass of Carbon Dioxide?

s

Relative Formula Mass = Mass ÷ Number of Moles Relative Formula Mass = 440 ÷ 10 =44                                                        Relative Formula Mass of Carbon Dioxide = 44