Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Problem of the Week Archive => Topic started by: Borek on January 07, 2013, 11:26:48 AM
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A flask filled with a mixture of oxygen and methane weights 67.5 g. The same flask filled with methane weights 67.0 g, and filled with oxygen - 69.0 g.
Is there enough oxygen for a complete combustion of the methane in the mixture?
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No
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No
Agreed.
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While I agree with the answer, anyone cares to explain?
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While I agree with the answer, anyone cares to explain?
[tex]
n=\frac{PV}{RT} \\
n_{CH_4} = n_{O_2} = n_{mix}=n \\
n M_{CH_4} + m_{flask}=67 \\
n M_{O_2} + m_{flask}=69 \\
n=\frac{1}{8} \\
m_{flask}=65 \\
67.5-65= \left ( x_{CH_4} M_{CH_4} + (1-x_{CH_4}) M_{O_2} \right ) \times \frac{1}{8} \\
x_{CH_4}=\frac{3}{4} \\
x_{O_2}=\frac{1}{4} \\
[/tex]
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Doing the maths another way the flask plus methane and oxygen would need to weigh at least 68.34g for complete combustion.
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Doing the maths another way the flask plus methane and oxygen would need to weigh at least 68.34g for complete combustion.
Glad. I get the same.
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BTW, was the fact that P and T were constant supposed to be obvious somehow? Or implied?
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Implied.
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Yes usually if Borek is being sneaky you can see it in the question. As this one was quite simply worded the only way to solve it was to assume P and T were unchanged for each.