## Determining Empirical formula

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Joined: Fri Sep 28, 2018 12:19 am

### Determining Empirical formula

Can someone help me start the problem of finding the empirical formula when given molecules instead of atoms. For example, the question asks to find the empirical formula given 1.811 g of CO2 and 0.3172 g of H2O. Thank you!

Chem_Mod
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### Re: Determining Empirical formula

You must figure out how much Carbon, Hydrogen, and oxygen you have in your original sample using principles of conservation of mass. See UA, TA office hours for detailed help.

Justin Haggard 1E
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### Re: Determining Empirical formula

You would need to find the moles of the entire molecules first, then use mole-to-mole ratios to find the moles of each element.

Start with the $CO_{2}$ molecule:

$1.811 g$ of $CO_{2}\cdot \frac{1 mole}{40.01 g}= 0.04526$ moles of $CO_{2}$

Now calculate moles of Carbon and Oxygen from the moles of $CO_{2}$:

$.04526$ moles of $CO_{2}\cdot \frac{1 mol C}{1 mol CO_{2}}= .04526 mol$ of Carbon

$.04526$ moles of $CO_{2}\cdot \frac{2 mol O}{1 mol CO_{2}}= .09052 mol$ of Oxygen

Then you would do the same for the $H_{2}O$ molecule, then add the moles of Oxygen together (from the $CO_{2}$ and $H_{2}O$ molecules) to get the total moles of Oxygen.

Then divide all of the moles of the elements by the smallest value to get whole number ratios, which would give you the empirical formula.

Hope this helps.

Jasmine Chow 1F
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### Re: Determining Empirical formula

You should first convert all your elements into moles. you then divide by the smallest mole value to get your empirical equation values. Last multiply if needed to get whole numbers.

uhedlund
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Joined: Wed Nov 15, 2017 3:03 am

### Re: Determining Empirical formula

First, you must convert the molecules to moles. Using molar ratios, you can find grams of each of the individual elements. Then convert the masses of the individual elements to moles, and proceed to divide by the smallest number of moles (ex: if H=2, O=4, and H=6 divide all by 2) to find Empirical formula.