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Question

1.84 g of a mixture of CaCO3 and MgCO3 was heated to a constant weight. The constant weight of the residue was found to be 0.96 g. calculate the percentage composition of the mixture.. (Ca=40,Mg=24,C=12,O=16)

A
CaCO3=54.34%, MgCO3=45.66%
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B
MgCO3=54.34%, CaCO3=45.66%
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C
CaCO3=45.11%, MgCO3=54.89%
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D
MgCO3=73.24%, CaCO3=26.76%
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Solution

The correct option is C CaCO3=45.11%, MgCO3=54.89%
We have,
CaCO3+MgCO3CaO+MgO+CO2

Mass of calcium carbonate = x g
Mass of magnesium carbonate = (1.84 - x) g
Mass of calcium oxide = y g
Mass of magnesium oxide = (0.96 - y)g

Applying POAC for Ca atoms,

Moles of Ca atoms in CaCO3 = moles of Ca atoms in CaO

1×moles of CaCO3=1× moles of CaO
Molar masses of calcium carbonate and calcium oxide are 100 and 56, respectively
x100=y56

Again applying POAC for Mg atoms,

Moles of Mg in MgCO3 = moles of Mg in MgO
1×Moles of MgCO3=1× moles of MgO

The molar mass of magnesium carbonate and magnesium oxide are 84 and 40 respectively.
1.84x84=0.96y40

From eqn (i) and (ii),
we get x = 0.83 g, y = 0.465 g
% of CaCO3=0.831.84×100=45.11%% of MgCO3=54.89%

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