How do you solve empirical formula calculations?

May 24, 2016

The explanation is given below.

Explanation:

Step 1 : Assume percentages as masses contained in a total of 100 grams.

Step 2 : Use $n = \frac{m}{M}$ formula to calculate the number of moles in each atom.
$n$ = number of moles
$m$ = mass of substance
$M$ = molar mass

Step 3 : Divide all number of moles by the smallest.

Step 4 : Round them to nearest whole numbers and assign as subscripts.

Example question: Calculate empirical formula of a compound that has 11% hydrogen and 89% oxygen.

Assume that 100 grams contains $11$ $g$ of $H$ and $89$ $g$ of $O$.

To move on to step 2, you also need to know the molar mass of $H$ and $O$. In that case, the molar mass of $H$ is $1.0$ $g$/$m o l$ and the molar mass of $O$ is $16.0$ $g$/$m o l$.

Now you can find the number of moles for each atom.
$n \left(H\right)$ = $\frac{11}{1.0}$ = $11$ moles
$n \left(O\right)$ = $\frac{89}{16.0}$ = $5.6$ moles

Step 3, multiply the number of moles by the smallest.
$H$ = $\frac{11}{5.6} = 2.00$
$O = \frac{5.6}{5.6} = 1.00$

*Has been rounded to nearest whole number

Step 4, assign them as subscripts.
Since there is 2 Hydrogen and and 1 Oxygen, you know that the empirical formula is ${H}_{2} O$.