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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

Title: Problem of the week - 07/01/2013
Post by: Borek on January 07, 2013, 11:26:48 AM: 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?
Title: Re: Problem of the week - 07/01/2013
Post by: DrCMS on January 07, 2013, 02:47:57 PM: No
Title: Re: Problem of the week - 07/01/2013
Post by: curiouscat on January 09, 2013, 04:47:12 AM: Quote from: DrCMS on January 07, 2013, 02:47:57 PM
No

Agreed.
Title: Re: Problem of the week - 07/01/2013
Post by: Borek on January 10, 2013, 05:32:07 AM: While I agree with the answer, anyone cares to explain?
Title: Re: Problem of the week - 07/01/2013
Post by: curiouscat on January 10, 2013, 07:18:51 AM: Quote from: Borek on January 10, 2013, 05:32:07 AM
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]
Title: Re: Problem of the week - 07/01/2013
Post by: DrCMS on January 10, 2013, 07:46:54 AM: Doing the maths another way the flask plus methane and oxygen would need to weigh at least 68.34g for complete combustion.
Title: Re: Problem of the week - 07/01/2013
Post by: curiouscat on January 10, 2013, 08:04:03 AM: Quote from: DrCMS on January 10, 2013, 07:46:54 AM
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.
Title: Re: Problem of the week - 07/01/2013
Post by: curiouscat on January 10, 2013, 08:50:05 AM: BTW, was the fact that P and T were constant supposed to be obvious somehow? Or implied?
Title: Re: Problem of the week - 07/01/2013
Post by: Borek on January 10, 2013, 10:46:14 AM: Implied.
Title: Re: Problem of the week - 07/01/2013
Post by: DrCMS on January 10, 2013, 11:22:40 AM: 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.