How can I find the molecular formula in this problem?

In summary, the student attempted to solve a problem that they believed was wrong, and was unable to do so because of incorrect numbers.
  • #1
nineteen
68
12

Homework Statement



In an organic compound "A" only C,H and O is present. 1.22g of A is completely burned and it gives 0.84g of CO2 and 0.54g of H2O. If the relative molecular mass of "A" is 123. Find the molecular formula of A.
[/B]
I have showed my attempt at solving this problem, but I think it is wrong. Please help me out here. After what I have done last, there are only a few more steps to find the empirical formula and then the molecular formula itself, but I think what I have done is wrong. Please try and correct me, I'd appreciate it a lot.

2. Homework Equations

no. of moles = Mass/Molar mass
n of the molecular formula = Molar mass/ Formula mass of the empirical formula

(I guess that those are the only equations)​

The Attempt at a Solution


[/B]
Mass of C in CO2 produced: (12/44) x 0.84g = 0.22g
Mass of O in CO2 produced: (32/44) x 0.84g = 0.61g

Mass of H in H2O produced: (2/18) x 0.54g = 0.06g
Mass of O in H2O produced : (16/18) x 0.54g = 0.48g

Total masses;
C = 0.22g
O = 1.09g
H = 0.06g

Molar ratios of elements;

C ----> (0.22/12) = 0.01
H ----> (0.06/1) = 0.06
O ----> (1.09/16) = 0.06

Dividing all by the smallest value;

C ----> 0.01/0.01 = 1
H ----> 0.06/0.01 = 6
O ----> 0.06/0.01 = 6
 
Last edited:
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  • #2
Hi 19,

nineteen said:
Molar ratios of elements
in what ? In A ?
 
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  • #3
BvU said:
Hi 19,

in what ? In A ?

Yes, the elements C,H and O...
By all means, I don't know whether this approach to the solution is correct or not.
 
  • #4
It is not. Your elements add up to 1.38 g. There is only 1.22 g of A
 
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  • #5
BvU said:
It is not. Your elements add up to 1.38 g. There is only 1.22 g of A

Yeah that's what. Can you please help me find the solution to this?
 
  • #6
Write the stoeciometric equation for the reaction ##C_xO_yH_z + ?? O_2 \rightarrow ?? CO_2 + ?? H_2O##
Three unknowns, so you need to find three equations.
 
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  • #7
BvU said:
Write the stoeciometric equation for the reaction ##C_xO_yH_z + ?? O_2 \rightarrow ?? CO_2 + ?? H_2O##
Three unknowns, so you need to find three equations.

Isn't there any other method to solve this, except for the stoichiometry method?
 
  • #8
You'll need it, I'm afraid. Express the ?? in terms of ##x, y ## and ##z##
 
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  • #9
BvU said:
You'll need it, I'm afraid. Express the ?? in terms of ##x, y ## and ##z##
Thank you very much!
 
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  • #10
The numbers here make no sense. Apart from anything else, an organic compound containing only C, H and O cannot have an odd RMM. Check that you've copied all the numbers correctly; if you have, I suspect a mistake in the question.
 
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  • #11
mjc123 said:
The numbers here make no sense. Apart from anything else, an organic compound containing only C, H and O cannot have an odd RMM. Check that you've copied all the numbers correctly; if you have, I suspect a mistake in the question.

Ok friend, I'll look into it.
 

Related to How can I find the molecular formula in this problem?

1. How do I determine the molecular formula from a chemical equation?

To determine the molecular formula from a chemical equation, you will need to know the molar mass of each element present in the compound. You can then use this information to calculate the empirical formula, which is the simplest whole number ratio of atoms in a compound. From there, you can use the molar mass of the compound to calculate the molecular formula.

2. What is the difference between empirical and molecular formula?

The empirical formula is the simplest whole number ratio of atoms in a compound, while the molecular formula is the actual number of atoms of each element present in a molecule. The molecular formula can be a multiple of the empirical formula, depending on the size of the molecule.

3. How do I find the molar mass of a compound?

To find the molar mass of a compound, you will need to know the molar mass of each element present in the compound. This can be found on the periodic table. Then, you can calculate the molar mass by multiplying the number of atoms of each element by its molar mass and adding them together.

4. Can the molecular formula be simplified?

Yes, the molecular formula can be simplified if it is a multiple of the empirical formula. To simplify, divide the subscripts of each element by the greatest common factor.

5. How can I confirm that my calculated molecular formula is correct?

To confirm the accuracy of your calculated molecular formula, you can use analytical techniques such as mass spectrometry or elemental analysis. These methods can determine the exact composition of a compound and confirm its molecular formula.

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