Question

One mole of a $\alpha$ emitter of half life equal to $4$ days was placed in a sealed tube for $8$ days, at $S.T.P$. volume of helium collected

• A. $22.4lit$
• B. $16.8lit$
• C. $11.2lit$
• D. $5.6lit$

Answer 1

Answer: (B)

$16.8lit$

This questions means to say that $\alpha$ emitted is actually the same as the Helium collected and we can safely assume that the amount of mole $\alpha$ emitted decays is the same amount of mole produced as Helium.
As, half-life is 4 days, so, after 2 half lives or 8 days, amount of $\alpha -particles$ or rather say Helium emitted
$=1-(\frac{1}{2}{)}^2=1-\frac{1}{4}=\frac{3}{4}mole$
So, at STP 1 mole means 22.4 lit
$\frac{3}{4}$ mole means 16.8 litres of gas collected.