Question

Two elements $X$ and $Y$ (atomic mass of $X=88\text{u}$,$Y=16\text{u}$) combine to give a compound that has $78.5\%$ of $X$. $1\text{g}$ of the gas occupies $0.1\text{L}$ at STP The formula of the compound is:

• A. $XY$
• B. $X_{2}Y$
• C. $X_2{Y}_2$
• D. $X_2{Y}_3$

Answer 1

The correct option is D $X_2{Y}_3$

Calculation of the empirical formula of the compound:
$\begin{array}{cccccc}\text{Element}& \text{\% in compound}& \text{Atomic mass}& \text{No. of moles}& \text{Simple ratio}& \text{Simple whole number}\\ X& 78.5& 88& \frac{78.5}{88}=0.89& \frac{0.89}{0.89}=1& 1\times 2=2\\ Y& 21.5& 16& \frac{24}{16}=1.34& \frac{1.34}{0.89}=1.5& 1.5\times 2=3\end{array}$
So, the empirical formula will be $X_2{Y}_3$.
Empirical formula mass $=2\times 8+16\times 3=224$
$1\text{g}$ of the substance occupies $0.1\text{L}$ volume at STP.
Molar mass of the compound $=\frac{1}{0.1}\times 22.4=224\text{g}$
Empirical formula mass $=$ molecular formula mass
Empirical formula $=$ molecular formula