# Homework Help: Molar mass to molecular formula

1. Sep 20, 2009

### iwin2000

1. The problem statement, all variables and given/known data
The empirical formula of a compound is C3H2O. If the molar mass of this compound is 192.8, what is its molecular formula?

1. C12H8O4
2. C7H4O
3. C9H6O3
4. C15H10O5

2. Relevant equations

3. The attempt at a solution

I'm having trouble with this question, I am not getting a whole number value for mass so I don't know what to do.

I did this: 3C->36g , 2H -> 2g , O -> 16 g, TOTAL: 54g

Compound: 192.8.
192.8/54 = 3.57.

Therefore, I guess it could be a or b as an answer, but I don't know what to do. Can someone help?

Last edited: Sep 20, 2009
2. Sep 20, 2009

### Staff: Mentor

Is it possible that C7H4O is the molecular formula?

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methods

3. Sep 20, 2009

### iwin2000

Yes that is one of the answers, is that the correct one?
Can you briefly explain how you got it please?

Thanks.

4. Sep 20, 2009

### symbolipoint

You want as close to a whole number quotienta as you can find. You only used one unit of "192.8". You could try other counts of "192.8", as long as you test whole number values.

Borek, somehow I do not like the choice of C7H4O. Can you support this choice?

5. Sep 20, 2009

### Staff: Mentor

You misunderstood my question, but I see now that what I posted could be confusing.

What I meant was: can this answer can be a correct one, if the emprical formula is C3H2O?

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methods

6. Sep 20, 2009

### iwin2000

Hmm, I guess I still don't understand what you're asking. Are you questioning the validity of the question, maybe it is wrong?
This is a general chemistry class I'm taking, I'm in my freshman year of college. The questions have been wrong a couple times before in previous assignments (and we've only had 2 previous assignments) so it may be a problem with the question.

I will email the professor for more information.
The assignment is due this Tuesday so I still have time.
Thanks for your help so far.

7. Sep 20, 2009

### Staff: Mentor

There is a simple rule that molecular formula must follow if empirical formula is known.

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8. Sep 21, 2009

### iwin2000

Just to give an answer to this question, my professor told me to round the ratio to the nearest whole number. Since it was 3.57 I should round it to 4, thus the correct answer would be C12H8O4.

9. Sep 21, 2009

### symbolipoint

That is not very close to a whole number. Can you find a ratio of formula units which gives something closer, much closer, to a whole number?

10. Sep 22, 2009

### burningbend

definitely a problem with the question itself. molecular formulas need to be exactly integer multiples of empirical formulas, and 3.5 isn't close enough to an integer value to count. tell them to rerun the mass spec data lol.

11. Sep 22, 2009

### iwin2000

Thanks guys, I'll keep it in mind for next time. Either way, I got the question correct, so you guys helped a lot.

My professor actually went on to say during next class that we could round up or down and she would count either as correct. Also she stated that the ratio was inaccurate, but those are the type of values we can expect to get in real life.

12. Sep 23, 2009

### symbolipoint

iwin2000 lastly wrote:
Oo-oo-oo-oo-ooh! Burningbend, Borek, what do you two think of that? Maybe people in real life need better instruments or need be more careful with their techniques?

13. Sep 23, 2009

### burningbend

someone's probably just an idiot and screwed something up along the way. mass specs are pretty accurate in giving molecular ions for species this small

14. Sep 23, 2009

### symbolipoint

Now, dig this!

choice 1: 216.196/54 = 4.0036 (did I do something wrong? used software calculator Win.)

choice 3: 162.147/54 = 3.0027

choice 4: 270.245/54 = 5.045

Unless I did something wrong, choice #1 or #3 seems to be the best. Distinguishing does not seem significant.

15. Sep 23, 2009

### burningbend

well yeah, that's the point, that when you divide the molecular weight by the empirical weight, you get an integer. the problem is the molecular weight given doesn't match any of the species listed.

16. Sep 23, 2009

### burningbend

now that i think about it, the bad mass is probably due to not purifying the sample enough and using an impure sample in combustion analysis.

17. Sep 23, 2009

### Staff: Mentor

Honestly, I have no idea what you are doing and what for. If you will use 54.0474 as molar mass of C3H2O you willl get numbers even closer to integers, but basically what you are doing is 3/1=3 or 4/1=4. Your starting point is experimentally detected 192.8, not molar masses of given formulas.

Note, that a lot depends on the method of determining molar mass, so 192.8 g/mol can be a honest result, one that needs refining. Imagine using ebuliometric or crioscopic method to determine molar mass of substance that dimerizes to some extent in the solution. You didn't know that beforehand, all you know is that molar mass is off and you have to deal with it somehow.

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methods

18. Sep 23, 2009

### Staff: Mentor

Nah, bad mass is by question design, I don't think these are real results

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