Question

Acetylene torches are used for welding. These torches use a mixture of acetylene gas, C2H2, and oxygen gas, O2 to produce the following combustion reaction:

2C2H2(g)+5O2(g)→4CO2(g)+2H2O(g)

Imagine that you have a 5.50 L gas tank and a 4.50 L gas tank. You need to fill one tank with oxygen and the other with acetylene to use in conjunction with your welding torch. If you fill the larger tank with oxygen to a pressure of 115 atm , to what pressure should you fill the acetylene tank to ensure that you run out of each gas at the same time? Assume ideal behavior for all gases.

Answer 1

2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g)

From the data volume of O2 gas = 5.5L

pressure of O2 gas = 115 atm

Assume room temperature = 298K

number of moles of O2 n = PV/RT

n = 115 * 5.5/(0.0821 * 298)

n = 25.8524 moles

5 moles of O2 requires 2 moles of C2H2

25.8524 moles of O2 requires 2/5*25.8524 moles of C2H2

                           = 10.341 moles of C2H2

volume of C2H2 gas = 4.5 L

From ideal gas equation,

P1V1/n1 = P2V2/n2 at same temperature,

115 * 5.5/25.8524 = P2 * 4.5/10.341

P2 = pressure of C2H2 gas = 56.222 atm.