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Posted by on Wednesday, August 7, 2013 at 11:46pm.

Can one calculate the number of moles of air? If so, calculate the number of moles in 1 kg of air (making sure your calculation, units and assumptions are fully explained); If not, explain why it is not possible to calculate the number of moles of air.

  • chemistry - , Thursday, August 8, 2013 at 12:10am

    Air is a mixture of several gases. But the most abundant is Nitrogen and Oxygen. Their mole percent in air is:
    N2 = 78%
    O2 = 21%
    Ar = 1%
    *Actually the 1% here is not entirely Argon, but Argon is the most abundant in that 1%.
    Note that these values are NOT EXACT, but is just an estimation. To calculate for number of moles of air given the mass of air, determine first the molar mass of air. To do this we multiply the mole percent of each by their respective molar masses:
    N2: 0.78 * 28 g/mol = 21.84 g N2 / mol air
    O2: 0.21 * 32 g/mol = 6.72 g O2 / mol air
    Ar: 0.01 * 40 g/mol = 0.4 g Ar / mol air
    Adding,
    21.84 + 6.72 + 0.4 = 28.96 g / mol air

    Now that we know the molar mass, we divide the 1 kg of air by the molar mass:
    1000 g air / (28.96 g / mol air)
    = 34.53 mol air

    Hope this helps~ :)

  • chemistry - , Thursday, August 8, 2013 at 1:36am

    That is excellent thank you. Now you have given me the answer working through it myself makes a lot more sense.

    Can you help me with the one below as well.

    A 25.0 mL solution of HCl has been spilt. It took 21.8 mL of saturated 2.5 M NaOH to neutralise the acid. What was the molar concentration of the acid?

  • chemistry - , Thursday, August 8, 2013 at 3:35am

    If the volume is split, then the amount of moles will be split as well. However, the mole to volume ratio will not change in a homogenous mixture. So, the molarity can be solved using MV=MV

    M=2.5M*(21.8mL/12.5mL)

    Answer contains 2 significant figures.

  • To Sarah--chemistry - , Thursday, August 8, 2013 at 1:44pm

    I worked that HCl/NaOH problem for you before you posted the air question.

  • To Devron--chemistry - , Thursday, August 8, 2013 at 1:48pm

    I think you misinterpreted Sarah's question. I believe spilt, in Sarah's culture, means spilled in ours (and not split). I think her question is just a "titration" question. My response is below at an earlier post.

  • chemistry=>Dr.Bob222 - , Thursday, August 8, 2013 at 6:17pm

    Okay!!!!!!!

  • chemistry=>Dr.Bob222 - , Thursday, August 8, 2013 at 6:39pm

    I was wondering why the individual ask the question when it seemed kind of straight forward. Sometimes I want to answer a question, but I am not sure what they are asking, or I'm not sure concerning the concept if I haven't done a particular type of problem in a while. So, I just don't respond, and I decide to let you or someone else handle the problem. I figured that some of the questions come from individuals that speak english as their second language, or are from people studying science in places outside the US because some of the questions that I see posted are not questions that you will typically see in a chem. science book in America (e.g. analytical chem. problem earlier last week with an apparatus on the right). But I answer the questions if I think I can, or I have an idea on how to solve them because science is science, and I like to help people learn science because science is fun.

  • chemistry - , Thursday, August 8, 2013 at 6:41pm

    I do mean spilled, thanks.

  • To Devron--chemistry - , Friday, August 9, 2013 at 2:38pm

    I have learned through experience that students in the UK use spilt while US uses spilled. Same thing for spelling colour or odour or sulphur etc. Canadian students spell the same as the British. I don't know if Sarah is Canadian but UK for sure. The aussies are UK also. For those strange analytical chemistry question, I agree. Some are clearly worded wrong but most of those who speak English use the British system; i.e., India, Pakistan, etc. Therefore, some of those look a little different for two reasons (1) English is a second language and sentence structure is not the best (2) what little English they do speak is British. Remember, however, that many of the U.S.students don't do much better. Many U.S. students are confused, too, and their sentence structure is so poor sometimes I have no idea what they've asked.

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