Question

$20.0$ mg of a magnesium carbonate sample decomposes on heating to give carbon dioxide and $8.0$ mg magnesium oxide. What will be the percentage purity of magnesium carbonate in the sample?

[Atomic weight of $Mg=24$]

• A. $60$
• B. $80$
• C. $75$
• D. $96$

Answer 1

Answer: (B)

$80$

$MgC{O}_3\to MgO+C{O}_2$

The molar masses of $MgC{O}_3$ and $MgO$ are $24+12+48=84$ g/mol and $24+16=40$ g/mol respectively.

Thus, $8$ g $MgO$ will be obtained from $\frac{84}{40}\times 8=16.8$ mg of $MgC{O}_3$.

The percentage purity of magnesium carbonate in the sample is $\frac{16.8}{20}\times 100=80\%$

Hence, the correct option is $B$