a sample of CaCo3 is 50% pure on heating 1.12L of Co2(al STP) is obtained.residue left is- a)7.8 b)3.8 c)2.8 d)8.9

CaCO3 decomposes as following reaction:

 
CaCO3 ? CaO + CO2

According to the reaction, 1 mol CaCO3 produces 1 mol CO2

 

At STP 1mol CO2 = 22.4L
We have 1.12L = 1.12/22.4 = 0.05 mol CO2

Therefore we had 0.05 mol CaCO3 to start. 
Molar mass CaCO3 = 100g/mol
Now, we had 0.05mol = 0.05*100 = 5g CaCO3
we started with 5g CaCO3 and 5g Impurity
Molar mass CaO = 56g/mol
Mass of CaO remaining = 5* 56/100 = 2.8g

Total mass residue = 5+2.8 = 7.8g
Choice 1 is correct.