Homework Help: chemistry

Posted by Joshua on Tuesday, October 28, 2008 at 12:34am.

I need to know if I have th right molecular formula for Sorbito

39.56g C

7.75g H

52.7g O

Empirical Formula is CH2O

30.026 amu

how I how calculate the molecular mass and how I figure out the melecular formula? please help

chemistry - Dr Russ, Tuesday, October 28, 2008 at 7:06am
Do you mean sorbitol?

If all the information that you have is the elemental compositions then you can only calculate the empirical formula. CH2O is NOT the correct empirical formula for sorbitol.

C H O
39.56% 7.75% 52.7%
39.56g 7.75g 52.7g
3.30mol 7.75 mol 3.29 mol
1 mol 2.34 mol 1 mol

which is not CH2O

C3H7O3 is the closest

In order to calculate the molecular formula you meed the molecular mass calculated by another means.
chemistry - DrBob222, Tuesday, October 28, 2008 at 11:26am
In other posts of this same compound, the rest of the problem was to calculate the molar mass of sorbitol from freezing point depression data. As Dr Russ notes, that information will provide the molar mass and from that you can determine the molecular formula.
chemistry - joshua, Tuesday, October 28, 2008 at 12:11pm
so the molecular formula will be?

c= 3.30/3.29=1, H=7.75/3.29=2, O=3.29/3.29=1

2(C3H7O3)= C6H17O6

Delta T= Kf * m

T= -1.86 C/m * 0.33m/g

T= o.613 C

am i correct?
chemistry - DrBob222, Tuesday, October 28, 2008 at 12:20pm
I don't remember the numbers for the freezing point depression part of the problem, but the molar mass works out to be about 182 or so; therefore, the multiplier you have of 2 is correct. You made a typo in multiplying; however, since 2(C3H7O3) = C6H14O6 (not the H17 you typed in).
chemistry - joshua, Tuesday, October 28, 2008 at 12:47pm
sorry it was a typo and yes they didn't ask for the freezing point thanks so much

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