# How can I find the molecular formula in this problem?

#### nineteen

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:
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Homework Helper

#### nineteen

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

Homework Helper
It is not. Your elements add up to 1.38 g. There is only 1.22 g of A

#### nineteen

It is not. Your elements add up to 1.38 g. There is only 1.22 g of A

#### BvU

Homework Helper
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.

#### nineteen

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

Homework Helper
You'll need it, I'm afraid. Express the ?? in terms of $x, y$ and $z$

#### nineteen

You'll need it, I'm afraid. Express the ?? in terms of $x, y$ and $z$
Thank you very much!!

#### mjc123

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.

#### nineteen

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.

"How can I find the molecular formula in this problem?"

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