Practice Midterm Question

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allyz1F
Posts: 58
Joined: Sat Jul 22, 2017 3:00 am

Practice Midterm Question

Postby allyz1F » Sat Nov 11, 2017 8:43 pm

I don't have the exactly worded question, but the problem was asking for the molecular formula of a compound that was 43.2% carbon, 6.35% hydrogen, and 50.4% oxygen. I am just not sure why I don't get the same result, but I think it has something to do with the carbon not resulting in a whole or multipliable number when it is divided by the least number of moles. But when you multiply it to round it off, it still results in the wrong molecular formula, so I'm just not sure what to do from there.

Karan Singh Lecture 3
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Joined: Fri Sep 29, 2017 7:07 am

Re: Practice Midterm Question

Postby Karan Singh Lecture 3 » Sat Nov 11, 2017 9:24 pm

Is the molar mass of the substance given? If so you could multiply all the percentages by the total and then divide by the molar mass of each particular substance. That will get you the molecular formula. Hope this helps.

Tanaisha Italia 1B
Posts: 55
Joined: Fri Sep 29, 2017 7:04 am

Re: Practice Midterm Question

Postby Tanaisha Italia 1B » Sat Nov 11, 2017 11:45 pm

Hi,
Is the answer by any chance C8H14O7? If it is, then you just need to find the number of moles of Carbon (grams/molar mass), divide it by the least number of moles, and multiply each answer by seven to get a whole number.

Brigitte Phung 1F
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Re: Practice Midterm Question

Postby Brigitte Phung 1F » Sun Nov 12, 2017 12:53 am

I agree with Karan! If the molar mass of the compound is given (which I think it was!), you can just multiply it by the percentages to find out how many grams each element would contribute to the total mass of one mole of the compound. You can then directly divide the mass of each element by their respective molar masses to get the moles of each element in the molecular formula. With this method, you do not need to divide by the smallest number of moles! Hope this helps.

Jesus Rodriguez 1J
Posts: 53
Joined: Sat Jul 22, 2017 3:00 am

Re: Practice Midterm Question

Postby Jesus Rodriguez 1J » Sun Nov 12, 2017 10:10 pm

If you are given the percentages of each element. You can just use percentages as grams because the empirical formula is relative to the number of atoms. You would then divide the number of grams by the molar mass of each element to get moles. Then divide by lowest number of moles that you got and until you got whole numbers for each mol. That would give you the empirical formula but since the question is asking for molecular formula, you divide the molar mass of the entire compound (this number should be given to you) by the molar mass of the compound in the empirical form. This would give the Factor that you would have to multiply each of the subscripts by in order to get your answer.


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