Question

A mixture of calcium and magnesium carbonates weighing 1.4 g was strongly heated until no further loss of weight was perceived. The residue weighed 0.76 g. What percentage of $MgC{O}_3$ was present in the mixture?

• A. 20.45
• B. 18.55
• C. 16.33
• D. 22.4

Answer 1

The correct option is A 20.45

$CaC{O}_3+MgC{O}_3\to CaO+MgO$

let mass of $CaC{O}_3$ = x g

and mass of $MgC{O}_3$ = $1.4-x$ g

moles of $CaC{O}_3$ = $\frac{x}{100}$ = moles $CaO$ produced

moles of $MgC{O}_3$ = $\frac{x}{84}$ = moles $MgO$ produced

mass of $CaO$ produced = moles * no. of moles = $\frac{\left(x\right)\ast 56}{100}$

mass of $MgO$ produced = moles * no. of moles = $\frac{(1.4-x)\ast 40}{100}$

so, total weight of product = $\frac{(1.4-x)\ast 56}{100}$ + $\frac{(1.4-x)\ast 40}{100}$ = 0.76

so, value of x = 1.356 g

percentage of $CaC{O}_3$ = $\frac{1.356}{1.4}\ast 100$ = 96.8 %

percentage of $MgC{O}_3$ = 3.14 %