Please help me solve this problem! If your answer is correct, I'll choose it as the best & you'll get points!

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Please help me solve this problem! If your answer is correct, I'll choose it as the best & you'll get points!

[From: ] [author: ] [Date: 12-05-19] [Hit: ]

68 = 14.) The formula can be written as C7 H9 N O2.5 . Since you have a decimal 2.5, you must multiply by 2 to make the formula contain only whole numbers.......

Number of moles of Oxygen = (Mass of Oxygen / Molar Mass of Oxygen) = (27.18 / 16) = 1.699 moles

3.) The smallest value is that of Nitrogen, 0.68. Divide the number of moles of all elements by this:

Carbon : 4.762 / 0.68 = approximately 7
Hydrogen: 6.16 / 0.68 = approximately 9
Oxygen: 1.699 / 0.68 = approximately 2.5
Nitrogen: 0.68 / 0.68 = 1

4.) The formula can be written as C7 H9 N O2.5 . Since you have a decimal 2.5, you must multiply by 2 to make the formula contain only whole numbers. So the empirical formula is C14 H18 N2 O5

Note that if the question had given you masses, rather than percentages, then you could have skipped to step 2 immediately.

Now suppose the question added:

Find the molecular formula of NutraSweet k nowing the molar mass of NutraSweet is 294.30 g/mol.

Here, you just have to find the molecular mass of the empirical formula and then see the ratio between the molecular mass given in the question and that molecular mass. Then, you multiply the empirical formula by that ratio.

So in this example, the molecular mass of the empirical formula is:

Molecular mass of C14 H18 N2 O5 = 14(12) + 18(1) + 2(14) + 5(16) = 294 g/mol .

The ratio is : Molecular Mass Given / Molecular Mass of Empirical Formula = 1/1 = 1.
Here it is clear that the two masses are equal so the empirical formula calculated is in fact the molecuar formula. However, had the ratio been 2 for example, you woud have had to multiply the empirical formula by 2 to obtain the molecular formula.

Hope this was helpful

Regards,
Sami

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I think the way they usually do it is by combusting a known mass of the compound (presuming it's some sort of organic compound, for example ethane) and measuring how much water and carbon dioxide it gives off (i.e. measuring the masses). From that you can work out the moles of carbon and hydrogen in the compound by the moles of carbon and hydrogen present in the carbon dixide and water, and the ratio of the moles of carbon and hydrogen will give you the ratio of them in the molecule you don't know the formula of, which in turn will give you its empirical formula. Then you use the compound's molecular mass (presuming you know it, which you do need) to work out the multiple of the empirical formula which it is e.g. CH3 empirical formula, molecular mass of 30 , 2xCH3 has a molecular mass of 30 so it must be ethane.
Hope I helped :)

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