Chemistry - Molecular Formula
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Chemistry - Molecular Formula

[From: ] [author: ] [Date: 12-08-02] [Hit: ]
364 g CO2 and 0.5583 g H2O. If the molar mass of the compound was determined to be approximately 84.09 g/mol, what is its molecular formula?I need help on this question.......
A 0.5211 g sample of a compound containing C, H, and N was burned to produce 1.364 g CO2 and 0.5583 g H2O. If the molar mass of the compound was determined to be approximately 84.09 g/mol, what is its molecular formula?

I need help on this question. I know you have to find the empirical formula first, then find "n" and multiply it to the empirical formula to get the molecular formula. But I don't know how to find the empirical formula with the CO2 and H2O. How do you get the moles of C, H, and N from those 2 compounds? Thanks!

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When combusting an organic material, C ends up as CO2 and H ends up as H2O. N can end up as either N2 or NO2, but that doesn't matter here.

1.364 g CO2 x (1 mole CO2 / 44.01 g CO2) = 0.03099 moles CO2

0.03099 moles CO2 x (1 mole C / 1 mole CO2) = 0.03099 moles C in the original sample.
0.03099 moles C x (12.01 g C / 1 mole C) = 0.3722 g C in the original sample.

0.5583 g H2O x (1 mole H2O / 18.02 g H2O) = 0.03098 moles H2O

0.03098 moles H2O x (2 moles H / 1 mole H2O) = 0.06196 moles H in the original sample.
0.06196 moles H x (1.008 g H / 1 mole H) = 0.0625 g H in the original sample.

Since the original sample contains only C, H, and N, then

g N = g sample - g C - g H = 0.5211 - 0.3722 - 0.0625 = 0.0864 g N
0.0864 g N x (1 mole N / 14.01 g N) = 0.00617 moles N in the original sample.

So the mole ratio between C, H, and N = 0.03099 moles C / 0.06196 moles H / 0.00617 moles N.

If we divide each of those values by the smallest (0.00617), we get a C/H/N mole ratio of 5/10/1.

So the empirical formula = C5H10N. The molar mass for this empirical formula = 5 C (5 x 12.01) + 10 H (10 x 1.008) + N (14.01) = 84.14 which is very close to the actual molar mass given (84.07). So the empirical formula and the molecular formula are the same, C5H10N. (i.e., n = 1).
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keywords: Molecular,Formula,Chemistry,Chemistry - Molecular Formula
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