Question

One mole of $A$ present in a closed vessel undergoes decay as:
${}_Z^{m}A⟶$ ${}_{Z-4}^{m-8}B+2\left({}_2^{4}He\right)$
What will be the volume of helium gas collected at STP after $20$ days $(t_{1/2}ofA=10days)$?

Answer 1

We know that,
$N=N_0{\left(\frac{1}{2}\right)}^n$ where, $N=$ remaining mole of $A$
$N=1{\left(\frac{1}{2}\right)}^2=\frac{1}{4}$
Number of decayed moles $=1-\frac{1}{4}=\frac{3}{4}$
Number of moles of helium formed $=2\times$ number of decayed moles of $A=2\times \frac{3}{4}=\frac{3}{2}$
Volume of helium at STP $=\frac{3}{2}\times 22.4=33.6$ litre