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 2.50 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

Assuming ideal behavior the equation to use is: PV = nRT,

Now, we know that 2 moles of acetylene reacts with 5 moles of O2, and if we know that oxygen takes the larger tank, then we just need to calculate the moles of O2 at that pressure, and then, by stechiometry, we can calculate the moles of acetylene and then, the pressure needed.

Assuming a temperature of 25 °C (298 K), let's calculate the moles of O2: n = PV/RT

n O2 = 115 * 7 / 0.0821*298 = 32.9 moles

n acet = 32.9 * 2/5 = 13.16 moles

P = 13.16 * 0.0821 * 298 / 2.5
P = 128.79 atm

Hope this helps

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