for question 1)
If we assume complete combustion, we get only CO2 and H2O as products. No CO for instance. Hence, the equilibrated reaction is:
2.C2H2 + 5.O2 -----> 4.CO2 + 2.H2O
We need 5 moles of oxygen for 2 moles of acetylene:
n(A) / 2 = n(O) / 5 which is equivalent to n(O) / n(A) = 5 / 2
where n(A) is the mole number of acetylene and n(O) the mole number of oxygen.
To finish this problem, we apply the gas law for both tanks:
P(O) x V(O) = n(O) x R x T and P(A) x V(A) = n(A) x R x T
If we assume both tanks are at the same temperature, then we can divide one equation by the other one:
P(O)V(O) / P(A)V(A) = n(O) / n(A)
R and T cancel out
but, we saw that n(O) / n(A) = 5/2, P(O)=115atm, V(O)=7L and V(A)=2.5L
then (115 x 7) / (P(A) x 2.5) = 5/2
P(A) = 128.8atm
Hope my explanation is clear and without mistake