First, we need to assume acetylene to adapt an ideal gas behavior. So, we can use the Ideal Gas Law:
PV = nRT
Given:
V = 42 L
T = 305 K
P = 780 torr = 1.026 atm
R = gas constant, 0.0821 L atm/ mol K
n = amount of gas in the canister
1.026 atm * 42 L = n * 0.0821 L atm/mol K * 305 K
n = 1.72 moles of gas