gaseous state

Initially ... put PV = nRT .... (1)

Finally .... put (P/2)V = (n₁+n₂)RT ... (2)

From (2) .... PV = 2(n₁+n₂)RT

Now solving (1) and (2) ... 2(n₁+n₂) = n ... (3)

U need to find ... (n - n₁)/n

3C₂H₂ --> C₆H₆

Let x moles react ...

3 moles of C2H2 gives 1 mole of C6H6 ...
Therefore x moles give x/3 moles of C6H6 ....

n-n1 = x

n2 = x/3

from (3) ...

n1 = x/3 ...

n = 2(x/3 + x/3)

n = 4x/3 ....

Hence reqd. ans is ...

4x/3 - x/34x/3 = 3/4 = 75%