Question

A compound contains equal masses of the elements A, B & C. If the gram atomic masses of A, B & C are 60 g, 20 g & 40 g respectively. Determine the molecular formula of the compound if the molecular mass is 720 g.

• A. $A_3{B}_{2}C$
• B. $A{B}_2{C}_3$
• C. $A_4{B}_{12}{C}_6$
• D. $A_6{B}_3{C}_2$

Answer 1

The correct option is C $A_4{B}_{12}{C}_6$

Let individual mass of A, B & C be $x$ gm
Mass Ratio is $A:B:C\Rightarrow x:x:x$
Mole Ratio is $A:B:C\Rightarrow \frac{x}{60}:\frac{x}{20}:\frac{x}{40}$
On simplification it becomes $2:6:3$
$\therefore EmpiricalFormulabecomes{A}_2{B}_6{C}_3$
Hence empirical formula mass = $2\times 60+6\times 20+3\times 40=360g$
We know that $MolecularFormula=EmpiricalFormula\times n$
where, $n=\frac{MolecularMass}{EmpiricalMass}=\frac{720}{360}=2$
$\therefore MolecularFormula=(A_2{B}_6{C}_3{)}_2$
i.e. $A_4{B}_{12}{C}_6$