Relative Atomic & Relative Formula Masses | Study Planner | Revision | Questions | TargetTeachTutors
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Answers to the Questions:
Question 1
Calculate Relative Formula Mass (Mr) of the following compounds and molecules;
Show each calculation step-by-step.
This question includes;
Simple Relative Formula Mass Calculation
Calculate the Relative Formula Mass for the following compound:
(Ar for H – 1, O – 16, C – 12, Mg – 24, N - 14)
i) H₂O
H – 2 x 1 = 2
O – 1 x 16 = 16
Mr for H2O = 2 + 16 = 18
ii) CH4
C – 1 x 12 = 12
H – 4 x 1 = 4
Mr for CH4 = 12 + 4 = 16
iii) CO2
C – 1 x 12 = 12
O - 2 x 16 = 32
Mr for CO2 = 12 + 32 = 44
iv) NH4
N – 1 x 14 = 14
H – 4 x 1 = 4
Mr for NH4 = 14 + 4 = 18
v) MgO
Mg – 1 x 24 = 24
O – 16 x 1 = 16
Mr for MgO = 24 + 16 = 40
Multi-Step Relative Formula Mass Calculation
i) CaCO3
Ca - 1 x 40 = 40
C - 1 x 12 = 12
O - 3 x 16 = 48
Mr CaCO3 = 40 + 12 + 48 = 100
ii) NaCO3
Na - 1 x 23 = 23
C - 1 x 12 = 12
O - 3 x 16 = 48
Mr NaNO3 = 23 + 12 +48 = 83
iii) LiOH
Li - 1 x 7 = 7
O - 1 x 16 = 16
H - 1 x 1 = 1
Mr LiOH = 7 + 16 + 1 = 24
iv) Cu(OH)2
Cu - 1 x 64 = 64
O - 2 x 16 = 32
H - 2 x 1 = 2
Mr Cu(OH)2 = 64 + 32 +2 = 98
v) NaNO3
Na - 1 x 23 = 23
N - 1 x 14 = 14
O 1 3 x 16 = 48
Mr NaNO3 = 23 + 14 + 48 = 85
Larger Compound Challenge for Relative Formula Mass Calculation
For a greater challenge, calculate the Relative Formula Mass for the following:
Ar - C – 12, H – 1, O – 16, Al – 27, S – 32, Ca – 40, N – 14)
i) C6H12O6
C – 6 x 12 = 72
H – 12 x 1 = 12
O – 6 x 16 = 96
Mr for C6H12O6 = 72 + 12 + 96 = 180
ii) C2H5OH
C – 2 x 12 = 24
H – 5 x 1 = 5
O – 1 x 16 = 16
H – 1 x 1 = 1
Mr for C2H5 OH = 24 + 5 + 16 + 1 = 46
iii) Al2(SO4)3
Al – 2 x 27 = 54
S – 3 x 32 = 96
O – 12 x 16 = 192
Mr for Al2(SO4)3 = 54 + 96 + 192 = 342
iv) Ca(OH)2
Ca – 1 x 40 = 40
O – 2 x 16 = 32
H – 2 x 1 = 2
Mr for Ca(OH)2 = 40 + 32 + 2 = 74
v) (NH4)2SO4
N – 2 x 14 = 28
H – 8 x 1 = 8
S – 2 x 32 = 64
O – 4 x 16 = 64
Mr for (NH4)2SO4 = 28 + 8+ 64 + 64 = 164
Questions 2, 3 & 4 are yours.
Good Luck!