Question

$C{u}_{2}S$ and $M_{2}S$ are isomorphous in which percentage of sulphur is $20.14$% and $12.94$% respectively. The atomic weight of M is:

[Atomic wt. of Cu = 63.5]

• A. 208
• B. 108
• C. 112
• D. 106

Answer 1

The correct option is B 108

Assume atomic weight of $S$ and $M$ to be $x$ and $y$ respectively.

Percentage of sulfur is $20.14$% and $12.94$% in $C{u}_{2}S$ and $M_{2}S$ respectively.

For $C{u}_{2}S$, 2 moles Cu = 1 mole S.

1g of $C{u}_{2}S$ contains 0.7986g Cu and 0.2014g S respectively.

Hence, $2\times \frac{0.7986}{63.5}=1\times \frac{0.2014}{x}$

1g of $M_{2}S$ contains 0.8706g M and 0.1294g S respectively.

Hence, $2\times \frac{0.8706}{y}=1\times \frac{0.1294}{x}$

Solving tow equation we get, $y=107.70\approx 108.$