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Calculation of Empirical Formulae.
Example1. A compound contains 75% carbon and 25% hydrogen. What is its empirical
formula?
Elements Carbon Hydrogen
Amount in question (% or mass) 75 25Atomic mass (periodic table) 12 1
Number of moles = Amount in question
Atomic mass
75
12
!.25
25
1
25
Mole ratio (divide eac number of moles b! te
smallest number of moles)
!.25
!.251
25
!.25 "
Empirical formula C "#
2. An o#ide of carbon contains 27% carbon.
What is its empirical formula?
Elements Carbon $#ygen
Amount in question (% or mass) 27
Atomic mass (periodic table) 12
Number of moles = Amount in question
Atomic mass
2712
2.25
Mole ratio (divide eac number of moles b! te
smallest number of moles)
2.25
2.25
1
2.25
Empirical formula
. &luorspar is made of calcium and fluorine. 'f 51% is calcium( calculate the
empirical formula.
Elements Calcium fluorine
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide eac number of moles b! te
smallest number of moles)
Empirical formula
)mpirical
formula
C"#
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". 2.7*g of aluminium is combined +ith 1*.!5g of chlorine. What is the empiricalformula of the ne+ substance?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide eac number of moles b! te
smallest number of moles)
Empirical formula
5. 1.!,g of iron is combined +ith *.",g of o#ygen. What is the empirical formula of
the ne+ compound ?
Elements
Amount in question (% or mass)
Atomic mass (periodic table)Number of moles = Amount in question
Atomic mass
Mole ratio (divide eac number of moles b! te
smallest number of moles)
Empirical formula
!. -altpetre is a potassium salt. 't is 1.% nitrogen (,.!% potassium and "7.5%
o#ygen. What is its empirical formula? /Hint0 the table needs an e#tra column butyou +or out eerything else in the same +ay3
Elements
Amount in question (% or mass)
Atomic mass (periodic table)
Number of moles = Amount in question
Atomic mass
Mole ratio (divide eac number of moles b! te
smallest number of moles)
Empirical formula
Extension$ is one as an extra t&ist to it'.
7.A hydrocarbon has ,*% carbon and 2* %hydrogen. a3 Calculate its empirical formula.
b3 When the mass of a single molecule of this compound +as analysed( it +as
found to hae a mass of *g. What must the true /molecular3 formula of thissubstance be?
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Calculation of 4eacting asses.
)#ample 1
Calculate the mass of iron sulphide produced +hen 5.!g of iron reacts
completely +ith e#cess sulphur .
Equation &e 6 - &e-
is means 1mole of &e reacts +ith 1mole of - to mae 1mole of &e-
"o& man!
moles are &e
usin
We are using 5.!g &e. 8umber of moles of iron being used is 5.! *.1 oles
5!
"o& man!
moles of
product &ill
be made
-ince 1mole of &e reacts +ith 1mole of - to mae 1mole of &e-
9hen *.1 moles of &e +ill produce *.1moles of &e-
Amount of
product
produced.
1 mole of &e- /5!623 ,,g
-o( *.1 mole of &e- +ill hae a mass of *.1# ,, *.*
Example +
"o& muc carbon &ould be needed to ma,e *.* of carbon dioxideEquation C 6 $2 C$2
is means ::mole of C maes 1mole of C$2
"o& man!
moles are &e
ma,in
We hae made ::g of C$2
1 mole of C$2 has a mass of :::::::::g
8umber of moles of C$2 that hae been made ass of C$2 made
ass of 1mole of C$2
"o& man!
moles of
product &ill
be made
-ince 1mole of C$2 is made from 1mole of C(9his means +e must hae used :::::: moles of C.
Amount of
reactant
used
ass of carbon 4elatie atomic mass of carbon # number of moles used
.Calcium carbonate /CaC$3 decomposes +hen heated to gie calcium o#ide
/Ca$3and carbon dio#ide /C$23
a3 Write a balanced e;uation for the reaction. b3Calculate the mass of calcium carbonate needed to mae 5!*g of calcium o#ide.
". Copper carbonate decomposes to gie copper o#ide and carbon dio#ide.
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a3 Write a balanced e;uation for the reaction. b3What mass of copper carbonate is needed to produce ,g of copper o#ide?
5.Ho+ much calcium o#ide is produced by heating 25 tonnes of calcium carbonate?
!.What mass of +ater is produced by completely burning 15g of butane?
2C"
H1*
6 1$2
,C$2
6 1*H2
$.
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