Question

A compound made up of two elements $X$ and $Y$ has an empirical formula${X_2}Y$. If the atomic weight of $X$ is $10$ and that of $Y$ is $5$and the compound has a vapour density $25$, find its molecular formula.

Answer 1

Molecular formulas give the kind and number of atoms of each element present in the molecular compound. In many cases, the molecular formula is the same as the empirical formula. The chemical formula will always be some integer multiple of the empirical formula.

Complete step by step answer:
`Empirical formula of the compound made by two elements $X$and $Y$ is: ${X_2}Y$
Atomic weight of $X$and atomic weight of $Y$are $10$and $5$respectively.
$\therefore$Empirical formula mass $= 2 \times 10 + 5 = 25$
The compound ${X_2}Y$has a vapour density $= 25$
We know that,
Molecular weight $=$ Mass of one atom of hydrogen divided by mass of one molecule of a substance​
Vapour density $=$ Mass of same volume of hydrogen divided by mass of a certain volume of gas or vapour​
So, Molecular weight $=$ $2 \times$ vapour density
$\therefore$Molecular mass $=$ $2 \times 25 = 50u$
n$=$ number of moles $=$ Molecular mass /empirical formula mass
So, the number of moles = $$$$\frac{{50}}{{25}} = 2
Molecular formula $=$ empirical formula $\times 2$
Molecular formula$=$ {X_2}Y$$$$ \times 2
Molecular formula$=$ ${X_4}{Y_2}$

$\therefore$The molecular formula will be: ${X_4}{Y_2}$

Note: The vapour density would show whether a gas is denser (more prominent than one) or less thick than air. The density has suggestions for holder stockpiling and personnel security—if a compartment can release a thick gas, its fume could sink and, if combustible, gather until it is at a concentration adequate for start. Regardless of whether not combustible, it could gather in the lower floor or level of a limited space and uproot air, conceivably introducing a suffocation peril to people entering the lower part of that space.