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How can I find the molecular formula in this problem?

53
10
1. The problem statement, all variables and given/known data

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.

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. Relevant 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)​

3. The attempt at a solution

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:
53
10
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.
 

BvU

Science Advisor
Homework Helper
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It is not. Your elements add up to 1.38 g. There is only 1.22 g of A
 
53
10
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?
 

BvU

Science Advisor
Homework Helper
12,021
2,645
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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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?
 

BvU

Science Advisor
Homework Helper
12,021
2,645
You'll need it, I'm afraid. Express the ?? in terms of ##x, y ## and ##z##
 

mjc123

Science Advisor
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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.
 
53
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.
Ok friend, I'll look into it.
 

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