Question

$39$ g of an alloy of aluminium and magnesium when heated with excess of dil. HCl forms magnesium chloride, aluminium chloride and hydrogen. The evolved hydrogen collected at $0^{\circ }C$ has a volume of $44.8$ litres at $1$ atm pressure. The composition of the aluminium by moles is :

• A. $66.6$%
• B. $56.7$%
• C. $58.9$%
• D. $63.7$%

Answer 1

Answer: (A)

$66.6$%

$1$ mole of magnesium will give $1$ mole of hydrogen.
$1$ mole of aluminium will give $1.5$ moles of hydrogen.
$44.8$ L of hydrogen at STP corresponds to $2$ moles of hydrogen. They can be obtained from a mixture of $0.5$ mole of Mg and $1$ mole of Al.
Hence, the mole percent of aluminium in the mixture is $\frac{1}{1+0.5}\times 100=66.67\%$