8. GCSE level exam
practise questions on deducing molecular/empirical formula (may use mole concept)
Type in formula without subscripts e.g. Fe3O4 even though it is correctly written as Fe3O4
(updated Mar 24th 2026)
0.55g of boron combined with 1.2g of oxygen. Deduce the empirical formula of boron oxide. (relative atomic masses, Ar: B = 11, O = 16) [efm-1]
B2O3
BO
B2O
BO2
1.35g of aluminium was burned in excess fluorine to form 4.2g of aluminium fluoride. Deduce the empirical formula of aluminium fluoride. (relative atomic masses, Ar: Al = 27, F = 19) [efm-2]
AlF
AlF3
Al3F
AlF2
3.4g of an unusual carbon oxide contained 1.8g of carbon. Deduce the empirical formula of the carbon oxide. (relative atomic masses, Ar: C=12, O=16) [efm-3]
CO
CO2
C3O2
C2O3
In an experiment 3.175g of copper combined with 0.400g of oxygen. What is the empirical formula of the copper oxide? (relative atomic masses, Ar: Cu = 63.5, O = 16) [efm-4]
CuO
CuO2
Cu2O2
Cu2O
8.875g of chlorine combined with 14.25g of fluorine. What is the formula of the chlorine fluoride? (relative atomic masses, Ar: Cl = 35.5, F = 19) [efm-5]
ClF3
Cl3F
ClF
ClF2
A gas formed by the combination of hydrogen and sulphur consited of 5.88% hydrogen and 94.12% sulphur. What is its empirical formula? (relative atomic masses, Ar: H = 1, S = 32) [efm-6]
HS
H2S
HS2
H2S2
5.08g of iodine combined with 1.6g of oxygen. What is the formula of the iodine oxide? (relative atomic masses, Ar: I = 127, O = 16) [efm-7]
IO5
I2O
I2O5
I2O3
2.8g of lithium was heated in a stream of air forming 6.0g of lithium oxide. What is its formula? (relative atomic masses, Ar: Li = 7, O =16) [efm-8]
LiO
LiO2
LiO3
Li2O
A iron-copper sulphide ore consisted of 30.5% iron, 34.6% copper, and 34.9% sulphur. What is the empirical formula of the ore? (relative atomic masses, Ar: Fe = 56, Cu = 63.5, S = 32) [efm-9]
FeCuS2
FeCuS
FeCu2S
Fe2CuS2
0.48g of magnesium combined with 3.20g of bromine. What is the formula of magnesium bromide? (relative atomic masses, Ar: Mg = 24, Br = 80) [efm-10]
Mg2Br
MgBr2
MgBr
MgBr3
0.7g of nitrogen combined with 5.325g of chlorine. Deduce the formula of nitrogen chloride. (relative atomic masses, Ar: N = 14, Cl = 35.5) [efm-11]
NCl
NCl2
NCl3
N3Cl
Fluorine oxide consists of 70.4% fluorine and 29.6% oxygen by mass. Deduce the formula of fluorine oxide. (relative atomic masses, Ar: F = 19, O = 16) [efm-12]
FO
FO2
FO3
F2O
47.6g of uranium combined with 22.8g of fluorine. What is the empirical formula of this uranium fluoride? (relative atomic masses, Ar: U = 238, F = 19) [efm-13]
UF6
UF3
UF2
UF4
10.35g of lead was heated in excess oxygen. If 11.95g of a lead oxide was formed, deduce its empirical formula. (relative atomic masses, Ar: Pb = 207, O = 16) [efm-14]
PbO
PbO2
Pb2O
PbO3
10.8g of aluminium combined with 19.2g of sulphur. What is the empirical formula of aluminium sulphide? (relative atomic masses, Ar: Al = 27, S = 32) [efm-15]
Al3S2
AlS3
Al2S3
AlS2
A hydrocarbon of molecular mass 30 consisted of 80% carbon and 20% hydrogen by mass. Deduce the molecular formula of the hydrocarbon. (relative atomic masses, Ar: C = 12, H = 1) [efm-16]
CH3
C2H3
CH18
C2H6
6g of carbon combines with 71g of chlorine. What is the formula of the carbon chloride? (relative atomic masses, Ar: C = 12, Cl = 35.5) [efm-17]
CCl4
C4Cl
CCl2
C2Cl
An organic compound consisted of 52.18% carbon, 13.04% hydrogen and 34.78% oxygen by mass. Deduce its empirical formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-18]
CH6O
C2H6O
C2H6O2
CH3O
An chlorinated hydrocabon molecule consisted of 37.21% carbon, 7.75% hydrogen and 55.04% chlorine by mass. Deduce its empirical formula. (relative atomic masses, Ar: C = 12, H = 1, Cl = 35.5) [efm-19]
C2H6Cl
CH3Cl
C2H5Cl
C2H4Cl2
A calcium mineral consisted of 29.4% calcium, 23.5% sulphur and 47.1% oxygen by mass. Calculate the empirical formula of the mineral. (relative atomic masses, Ar: Ca = 40, S = 32, O = 16) [efm-20]
CaSO2
CaSO3
Ca2SO4
CaSO4
A salt consisted of 27.06% sodium, 16.47% nitrogen and 56.47% oxygen by mass. Deduce the formula of the salt. (relative atomic masses, Ar: Na = 23, N = 14, O = 16) [efm-21]
NaNO3
NaNO2
Na2NO3
NaNO
A brominated hydrocarbon consisted of 12.77% carbon, 2.13% hydrogen and 85.10% bromine by mass. If its molecular mass is 188, deduce the molecular formula of the molecule. (relative atomic masses, Ar: C = 12, H = 1, Br =80) [efm-22]
CH2Br
C2H4Br2
CH16Br2
C2H6Br2
It was found that 24g of carbon was combined with 5g of hydrogen in a hydrocarbon. If its molecular mass was 58, what is its molecular formula? (relative atomic masses, Ar: C = 12, H = 1) [efm-23]
C2H5
C3H8
C4H10
C3H22
A hydrocarbon consisted of 92.3% carbon and 7.7% hydrogen. If its molecular mass is 78, work out its molecular formula. (relative atomic masses, Ar: C = 12, H = 1) [efm-24]
C5H18
CH
CH6
C6H6
A carbohydrate on analysis consisted of 40.00% carbon, 6.67% hydrogen and 53.33% oxygen. If its molecular mass is 180, deduce the molecular formula of the carbohydrate. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-25]
C6H12O6
CH2O
C5H8O7
C3H6O3
7.75g of an alcohol consisted of 3.00g carbon, 0.75g hydrogen and 4.00g oxygen. If its molecular mass is 62, work out the molecular formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-26]
C3H10O
C2H6O2
CH3O
C2H6O
An alcohol consisted of 39.13% carbon, 8.70% hydrogen and 52.17% oxygen. If its molecular mass is 92, deduce its molecular formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-27]
CH2O
C3H6O3
C3H8O3
C2H4O4
In an experiment 6g of carbon combined with 19g of fluorine. Calculate the molecular formula of the compound formed if the molecular mass of it is 100. (relative atomic masses, Ar: C = 12, F = 19) [efm-28]
CF2
C4F2
CF4
C2F4
A compound of hydrogen and oxygen was made up of 0.5g hydrogen and 8g of oxygen. What is its empirical formula? (relative atomic masses, Ar: H = 1, O = 16) [efm-29]
HO
H2O2
H2O
HO2
A lithium compound consisted of 7g of lithium combined with 16g of sulphur and 32g of oxgen. What is the empirical formula of the compound? (relative atomic masses, Ar: Li = 7, S = 32, O = 16) [efm-30]
LiSO2
Li2SO4
LiSO4
Li2SO3
58g of a sodium phosphate salt consisted of 34.5g of sodium, 15.5g of phosphorus and 32.0g of oxygen. What is the simplest formula of the compound? (relative atomic masses, Ar: Na = 23, P = 31, O = 16) [efm-31]
Na4PO3
Na2PO4
Na3PO4
Na2PO3
A sodium compound consisted of 46g of sodium combined with 254g of iodine and 128g of oxygen. What is the compounds empirical formula? (relative atomic masses, Ar: Na = 23, I = 127, O = 16) [efm-32]
NaIO3
NaIO2
Na2IO4
NaIO4
16g of copper, after heating in oxygen, formed 20g of a copper oxide. What is the formula of the oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-33]
CuO
CuO2
Cu2O
CuO3
An oxide of carbon consisted of 3g of carbon combined with 4g of oxygen. What is the formula of the oxide? (relative atomic masses, Ar: C = 12, O = 16) [efm-34]
CO2
CO
C2O4
C2O
1.95g of potassium combined with 4.00g of bromine. What is the empirical formula of the compound? (relative atomic masses, Ar: K = 39, Br = 80) [efm-35]
K2Br
KBr2
KBr
KBr3
5g of calcium on heating in oxygen formed 7g of calcium oxide. What is the simplest formula of calcium oxide? (relative atomic masses, Ar: Ca = 40, O = 16) [efm-36]
CaO2
Ca2O
CaO3
CaO
13g of zinc combined with 6.4g of sulphur. What is the empirical formula of zinc sulphide? (relative atomic masses, Ar: Zn = 65, S = 32) [efm-37]
ZnS
ZnS2
Zn2S
ZnS3
An oxide of copper contained 80% copper and 20% oxygen by mass. What is the formula of the copper oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-38]
Cu2O
CuO
CuO2
CuO3
If magnesium oxide consists of 60% magnesium and 40% oxygen, what is its simplest formula? (relative atomic masses, Ar: Mg = 24, O = 16) [efm-39]
Mg2O
MgO2
MgO
MgO3
2.3g of sodium combined with 1.6g of sulphur. What is the formula of sodium sulphide? (relative atomic masses, Ar: Na = 23, S = 32) [efm-40]
NaS
NaS2
NaS3
Na2S
In an experiment 1.28g of copper combined with 0.16g of oxygen. What is the formula of the copper oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-41]
Cu2O
CuO
CuO2
CuO3
13.8g of sodium combined with 6.2g of phosphorus. What is the formula of sodium phosphide? (relative atomic masses, Ar: Na = 23, P = 31) [efm-42]
NaP
Na3P
NaP2
Na2P
10g of calcium combined with 40g of bromine. What is the formula of calcium bromide? (relative atomic masses, Ar: Ca = 40, Br = 80) [efm-43]
CaBr
Ca2Br
CaBr2
CaBr3
22g of boron combined with 114g of fluorine. What is the empirical formula of boron fluoride? (relative atomic masses, Ar: B = 11, F = 19) [efm-44]
BF
BF2
B3F
BF3
1.2g of titanium, after heating in oxygen, formed 2.0g of a titanium oxide. What is the empirical formula for the titanium oxide? (relative atomic masses, Ar: Ti = 48, O = 16) [efm-45]
TiO2
TiO
Ti2O
TiO3
0.800g of magnesium reacted with 8.47g of iodine. Calculate the empirical formula for magnesium iodide. (relative atomic masses, Ar: Mg = 24, I = 127) [efm-46]
MgI
MgI2
Mg2I
MgI3
0.55g of boron combined with 1.2g of oxygen. Deduce the empirical formula of boron oxide. (relative atomic masses, Ar: B = 11, O = 16) [efm-1]
1.35g of aluminium was burned in excess fluorine to form 4.2g of aluminium fluoride. Deduce the empirical formula of aluminium fluoride. (relative atomic masses, Ar: Al = 27, F = 19) [efm-2]
3.4g of an unusual carbon oxide contained 1.8g of carbon. Deduce the empirical formula of the carbon oxide. (relative atomic masses, Ar: C = 12, O = 16) [efm-3]
In an experiment 3.175g of copper combined with 0.400g of oxygen. What is the empirical formula of the copper oxide? (relative atomic masses, Ar: Cu = 63.5, O = 16) [efm-4]
8.875g of chlorine combined with 14.25g of fluorine. What is the formula of the chlorine fluoride? (relative atomic masses, Ar: Cl = 35.5, F = 19) [efm-5]
A gas formed by the combination of hydrogen and sulphur consited of 5.88% hydrogen and 94.12% sulphur. What is its empirical formula? (relative atomic masses, Ar: H = 1, S = 32) [efm-6]
5.08g of iodine combined with 1.6g of oxygen. What is the formula of the iodine oxide? (relative atomic masses, Ar: I = 127, O = 16) [efm-7]
2.8g of lithium was heated in a stream of air forming 6.0g of lithium oxide. What is its formula? (relative atomic masses, Ar: Li = 7, O =16) [efm-8]
A iron-copper sulphide ore consisted of 30.5% iron, 34.6% copper, and 34.9% sulphur. What is the empirical formula of the ore? (relative atomic masses, Ar: Fe = 56, Cu = 63.5, S = 32) [efm-9]
0.48g of magnesium combined with 3.20g of bromine. What is the formula of magnesium bromide? (relative atomic masses, Ar: Mg = 24, Br = 80) [efm-10]
0.7g of nitrogen combined with 5.325g of chlorine. Deduce the formula of nitrogen chloride. (relative atomic masses, Ar: N = 14, Cl = 35.5) [efm-11]
Fluorine oxide consists of 70.4% fluorine and 29.6% oxygen by mass. Deduce the formula of fluorine oxide. (relative atomic masses, Ar: F = 19, O = 16) [efm-12]
47.6g of uranium combined with 22.8g of fluorine. What is the empirical formula of this uranium fluoride? (relative atomic masses, Ar: U = 238, F = 19) [efm-13]
10.35g of lead was heated in excess oxygen. If 11.95g of a lead oxide was formed, deduce its empirical formula. (relative atomic masses, Ar: Pb = 207, O = 16) [efm-14]
10.8g of aluminium combined with 19.2g of sulphur. What is the empirical formula of aluminium sulphide? (relative atomic masses, Ar: Al = 27, S = 32) [efm-15]
A hydrocarbon of molecular mass 30 consisted of 80% carbon and 20% hydrogen by mass. Deduce the molecular formula of the hydrocarbon. (relative atomic masses, Ar: C = 12, H = 1) [efm-16]
6g of carbon combines with 71g of chlorine. What is the formula of the carbon chloride? (relative atomic masses, Ar: C = 12, Cl = 35.5) [efm-17]
An organic compound consisted of 52.18% carbon, 13.04% hydrogen and 34.78% oxygen by mass. Deduce its empirical formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-18]
An chlorinated hydrocabon molecule consisted of 37.21% carbon, 7.75% hydrogen and 55.04% chlorine by mass. Deduce its empirical formula. (relative atomic masses, Ar: C = 12, H = 1, Cl = 35.5) [efm-19]
A calcium mineral consisted of 29.4% calcium, 23.5% sulphur and 47.1% oxygen by mass. Calculate the empirical formula of the mineral. (relative atomic masses, Ar: Ca = 40, S = 32, O = 16) [efm-20]
A salt consisted of 27.06% sodium, 16.47% nitrogen and 56.47% oxygen by mass. Deduce the formula of the salt. (relative atomic masses, Ar: Na = 23, N = 14, O = 16) [efm-21]
A brominated hydrocarbon consisted of 12.77% carbon, 2.13% hydrogen and 85.10% bromine by mass. If its molecular mass is 188, deduce the molecular formula of the molecule. (relative atomic masses, Ar: C = 12, H = 1, Br =80) [efm-22]
It was found that 24g of carbon was combined with 5g of hydrogen in a hydrocarbon. If its molecular mass was 58, what is its molecular formula? (relative atomic masses, Ar: C = 12, H = 1) [efm-23]
A hydrocarbon consisted of 92.3% carbon and 7.7% hydrogen. If its molecular mass is 78, work out its molecular formula. (relative atomic masses, Ar: C = 12, H = 1) [efm-24]
A carbohydrate on analysis consisted of 40.00% carbon, 6.67% hydrogen and 53.33% oxygen. If its molecular mass is 180, deduce the molecular formula of the carbohydrate. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-25]
7.75g of an alcohol consisted of 3.00g carbon, 0.75g hydrogen and 4.00g oxygen. If its molecular mass is 62, work out the molecular formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-26]
An alcohol consisted of 39.13% carbon, 8.70% hydrogen and 52.17% oxygen. If its molecular mass is 92, deduce its molecular formula. (relative atomic masses, Ar: C = 12, H = 1, O = 16) [efm-27]
In an experiment 6g of carbon combined with 19g of fluorine. Calculate the molecular formula of the compound formed if the molecular mass of it is 100. (relative atomic masses, Ar: C = 12, F = 19) [efm-28]
A compound of hydrogen and oxygen was made up of 0.5g hydrogen and 8g of oxygen. What is its empirical formula? (relative atomic masses, Ar: H = 1, O = 16) [efm-29]
A lithium compound consisted of 7g of lithium combined with 16g of sulphur and 32g of oxgen. What is the empirical formula of the compound? (relative atomic masses, Ar: Li = 7, S = 32, O = 16) [efm-30]
58g of a sodium phosphate salt consisted of 34.5g of sodium, 15.5g of phosphorus and 32.0g of oxygen. What is the simplest formula of the compound? (relative atomic masses, Ar: Na = 23, P = 31, O = 16) [efm-31]
A sodium compound consisted of 46g of sodium combined with 254g of iodine and 128g of oxygen. What is the compounds empirical formula? (relative atomic masses, Ar: Na = 23, I = 127, O = 16) [efm-32]
16g of copper, after heating in oxygen, formed 20g of a copper oxide. What is the formula of the oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-33]
An oxide of carbon consisted of 3g of carbon combined with 4g of oxygen. What is the formula of the oxide? (relative atomic masses, Ar: C = 12, O = 16) [efm-34]
1.95g of potassium combined with 4.00g of bromine. What is the empirical formula of the compound? (relative atomic masses, Ar: K = 39, Br = 80) [efm-35]
5g of calcium on heating in oxygen formed 7g of calcium oxide. What is the simplest formula of calcium oxide? (relative atomic masses, Ar: Ca = 40, O = 16) [efm-36]
13g of zinc combined with 6.4g of sulphur. What is the empirical formula of zinc sulphide? (relative atomic masses, Ar: Zn = 65, S = 32) [efm-37]
An oxide of copper contained 80% copper and 20% oxygen by mass. What is the formula of the copper oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-38]
If magnesium oxide consists of 60% magnesium and 40% oxygen, what is its simplest formula? (relative atomic masses, Ar: Mg = 24, O = 16) [efm-39]
2.3g of sodium combined with 1.6g of sulphur. What is the formula of sodium sulphide? (relative atomic masses, Ar: Na = 23, S = 32) [efm-40]
In an experiment 1.28g of copper combined with 0.16g of oxygen. What is the formula of the copper oxide? (relative atomic masses, Ar: Cu = 64, O = 16) [efm-41]
13.8g of sodium combined with 6.2g of phosphorus. What is the formula of sodium phosphide? (relative atomic masses, Ar: Na = 23, P = 31) [efm-42]
10g of calcium combined with 40g of bromine. What is the formula of calcium bromide? (relative atomic masses, Ar: Ca = 40, Br = 80) [efm-43]
22g of boron combined with 114g of fluorine. What is the empirical formula of boron fluoride? (relative atomic masses, Ar: B = 11, F = 19) [efm-44]
1.2g of titanium, after heating in oxygen, formed 2.0g of a titanium oxide. What is the empirical formula for the titanium oxide? (relative atomic masses, Ar: Ti = 48, O = 16) [efm-45]
0.800g of magnesium reacted with 8.467g of iodine. Calculate the empirical formula for magnesium iodide.? (relative atomic masses, Ar: Mg = 24, I = 127) [efm-46]
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0.55g of boron combined with 1.2g of oxygen. Deduce the empirical formula of boron oxide.