Acetylene torches are used for welding. These torches use a mixture of acetylene

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 7.00 L gas tank and a 3.50 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.

Explanation / Answer

consider the given reaction

2 C2H2 + 5 02 --> 4 C02 + 2 H20

we can see that

moles of 02 required = 2.5 x moles of C2H2 taken

moles of 02 / moles of C2H2 = 2.5

n 02 / n C2H2 = 2.5

n2 / n1 = 2.5

now

for ideal gas PV = nRT

n = PV / RT

so

(P2V2/RT2) / (P1V1/RT1) = 2.5

here

temperature is constant so T1 = T2

so

P2V2 / P1V1 = 2.5

given

V2 = volume of larger tank in which oxygen is filled = 7

V1 = 3.5

P2 = tank with oxygen = 115

so

115 x 7 / ( P1 x 3.5) = 2.5

P1 = 92

so

the pressure in the acetylene tank should be 92 atm

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