CALCULATIONS FOR SOLIDS
In order to calculate the number of moles of a solid
substance the following equation must be used:
n= m
M
where:
n = number of moles (mol)
m = mass of solid (g)
M = relative atomic mass/molar mass (g.mol-1)
Question 1 – How many moles of calcium carbonate
(CaCO3) are there in 20 g?
n = m/M
= 20/100
= 0.20 mol
M(CaCO3) = 40 + 12 + 3 × 16
= 100 g.mol-1
Question 2 – What is the mass of 0.4 moles of sodium
chloride (NaCl)?
m=n×M
= 0.4 × 58.5
= 23.4 g
M(NaCl) = 23 + 35.5
= 58.5 g.mol-1
Question 3 – What mass of lead nitrate is required to
react with 41.5 g of potassium iodide?
They react as follows:
Pb(NO3)2
+
n(KI) = m/M
= 41.5/166
= 0.25 mol
2KI
→
PbI2
+
M(KI) = 39 + 127
= 166 g.mol-1
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2KNO3
n(Pb(NO3)2) : n(KI) = 1 : 2
so n(Pb(NO3)2) = 0.125 mol
m(Pb(NO3)2) = n × M
M(Pb(NO3)2) = 207 + 2 × 14
= 0.125 × 331
+ 6 × 16
= 41.4 g
= 331 g.mol-1
Question 4 – Using the same information as Question 3,
what mass of lead iodide is produced?
n(PbI2) : n(Pb(NO3)2) = 1 : 1 so n(PbI2) = .125 mol
m(PbI2) = n × M
= 0.125 × 461
= 57.6 g
M(PbI2) = 207 + 2 × 127
= 461 g.mol-1
PRACTICE QUESTIONS
1.
Calculate the molar mass of the following
compounds:
(a) Glucose – C6H12O6
(b) Sulfuric Acid – H2SO4
(c) Ammonia Gas – NH3
(d) Iron Oxide (Rust) – Fe2O3
2.
Calculate the number of moles of the following:
(a) 23.8 g of Octane (C8H18)
(b) 107.3 g of Hydrochloric Acid (HCl)
(c) 36.8 g of Carbon Dioxide (CO2)
(d) 5.9 g of Ethanol (C2H5OH)
3.
Calculate the mass of the following:
(a) 3.5 mol of Phosphoric Acid (H3PO4)
(b) 0.75 mol of Magnesium Bromide (MgBr2)
(c) 1.58 mol of Silver Sulfide (Ag2S)
(d) 10.3 mol of Copper Sulfate (CuSO4)
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