What Is the Molecular Formula of C3H2O with a Molar Mass of 192.8?

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Discussion Overview

The discussion revolves around determining the molecular formula of a compound with an empirical formula of C3H2O and a given molar mass of 192.8 g/mol. Participants explore various potential molecular formulas and the implications of the empirical formula in relation to the molar mass.

Discussion Character

  • Homework-related, Technical explanation, Debate/contested

Main Points Raised

  • One participant calculates the empirical formula mass as 54 g and finds a ratio of 3.57 when dividing the molar mass by this value, leading to uncertainty about the correct molecular formula.
  • Some participants propose C7H4O as a possible molecular formula, questioning its validity given the empirical formula.
  • Another participant suggests that rounding the ratio to the nearest whole number could lead to C12H8O4 being the correct answer, but this raises further questions about the accuracy of the ratio.
  • Concerns are raised about the validity of the question itself, with one participant suggesting that molecular formulas must be exact integer multiples of empirical formulas, implying that a ratio of 3.5 is problematic.
  • Several participants discuss the potential issues with the experimental determination of molar mass, including the possibility of using impure samples or inaccuracies in measurement techniques.
  • Another participant mentions that the professor indicated rounding could be acceptable, which adds to the confusion regarding the correct approach to the problem.
  • One participant calculates various ratios for the proposed molecular formulas, suggesting that choices 1 and 3 seem to yield the best results, but questions remain about the accuracy of the given molecular weights.
  • There is a suggestion that the discrepancies in molar mass could stem from the method of analysis used, indicating that the results may need refinement.

Areas of Agreement / Disagreement

Participants express differing opinions on the validity of the proposed molecular formulas and the appropriateness of rounding ratios. There is no consensus on the correct molecular formula, and the discussion remains unresolved regarding the implications of the empirical formula and the accuracy of the molar mass.

Contextual Notes

Participants note that the determination of molar mass can be influenced by various factors, including sample purity and the methods used for measurement. The discussion highlights the complexity of interpreting empirical and molecular formulas in the context of experimental data.

iwin2000
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Homework Statement


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

Homework Equations


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:
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Is it possible that C7H4O is the molecular formula?

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

Thanks.
 
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?
 
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
 
Borek said:
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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chemical calculators - buffer calculator, concentration calculator
www.titrations.info - all about titration methods

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

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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.
 
iwin2000 said:
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.

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
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
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
iwin2000 lastly wrote:
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.

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
symbolipoint said:
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?

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
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
symbolipoint said:
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.

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
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
symbolipoint said:
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.

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
burningbend said:
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.

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

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