9 Percentage by Mass
Example One
What is the percentage by mass of carbon in sodium hydrogencarbonate, NaHCO3?
RFM (NaHCO3) = RAM (Na) + RAM (H) + RAM (C) + 3 x RAM (O)
= 23 + 1 + 12 + 48
= 84
percentage by mass = RAM (C) / RFM (NaHCO3) x 100%
= 12/84 x 100%
= 14.3%
Example Two Sometimes there are two or
What is the percentage by mass of magnesium in magnesium more atoms of an element in
nitride, Mg2N3? the formula. For example,
there are two magnesium
RFM (Mg2N3) = 2 x RAM (Mg) + 3 x RAM (N) atoms in each magnesium
= 48 + 42 nitride unit. We therefore
= 90 have to multiply by two in
the second step.
percentage by mass = 2 x RAM (Mg) / RFM (Mg2N3) x 100%
= 48/90 x 100%
= 53.3%
Example Three
What is the percentage by mass of water in hydrated copper(II) sulfate, CuSO4.5H2O?
RFM (CuSO4.5H2O) = RAM (Cu) + RAM (S) + 4 x RAM (O) + 5 x (2 x RAM (H) + RAM (O))
= 64 + 32 + 64 + (5 x 18) Hydrated salts contain water of crystallization. You can work
= 160 + 90 out the percentage by mass of water in hydrated salts by using
the relative molecular mass of water in the second step.
= 250
percentage by mass = 5 x RMM (H2O) / RFM (CuSO4.5H2O) x 100%
= 90/250 x 100%
= 36%
1) Calculate the percentage by mass of:
a) chlorine in calcium chloride, CaCl2
b) iron in iron(II) oxide, FeO
c) sulfur in sodium sulfate, Na2SO4
d) sulfur in sodium thiosulfate, Na2S2O3
e) calcium in calcium hydrogencarbonate, Ca(HCO3)2
f) chlorine in poly(chloroethene) (PVC), (CH2CHCl)n
g) chromium in potassium dichromate(VI), K2Cr2O7
h) water in hydrated sodium ethanoate, NaCO2CH3.3H2O
i) water in hydrated sodium carbonate, Na2CO3.10H2O
j) magnesium in hydrated magnesium sulfate, MgSO4.7H2O

Fertilisers
Fertilisers provide nitrogen, N, phosphorus, P and
potassium, K, for healthy plant growth. When
farmers apply fertilisers to their crops, they need
to know exactly what mass of nitrogen, phosphorus
and potassium have been added. Chemists use
percentage by mass calculations to label the
fertilizer sacks.
2) Calculate the percentage by mass of potassium in the following fertilisers:
a) potassium chloride, KCl
b) potassium nitrate, KNO3
3) Calculate the percentage by mass of phosphorus in the following fertilisers:
a) phosphorus pentoxide, P2O5
b) calcium dihydrogenphosphate, Ca(H2PO4)2
4) Calculate the percentage by mass of nitrogen
in the following fertilisers:
a) ammonia, NH3
b) sodium nitrate, NaNO3
c) potassium nitrate, KNO3
d) ammonium nitrate, NH4NO3
e) ammonium phosphate, (NH4)3PO4
f) urea, (NH2)2CO
Answers
1. Calculate the percentage by mass:
(a) 64.0%, (b) 77.8%, (c) 22.5%, (d) 40.5%, (e) 24.7%, (f) 56.8%, (g) 35.4%, (h) 39.7%, (i) 62.9%, (j) 9.8%
2. Calculate the percentage by mass of potassium:
(a) 52.3%, (b) 38.6%
3. Calculate the percentage by mass of phosphorus:
(a) 43.7%, (b) 26.5%
4. Calculate the percentage by mass of nitrogen:
(a) 82.4%, (b) 16.5%, (c) 13.9%, (d) 35%, (e) 28.2%, (f) 46.7%