An empirical formula is a formula in which the minimal ratio of all the constituents of each compound is written in the formula. In order to change an empirical formula to a molecular formula the ratio between the molecular weight and the empirical weight would be found and then the empirical formula would be multiplied with this ratio.
- Start with the number of grams of each element, given in the problem.
If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.
- Convert the mass of each element to moles using the molar mass from the periodic table.
- Divide each mole value by the smallest number of moles calculated.
- Round to the nearest whole number. This is the mole ratio of the elements and is represented by subscripts in the empirical formula.
If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same factor to get the lowest whole number multiple.
e.g. If one solution is 1.5, then multiply each solution in the problem by 2 to get 3.
e.g. If one solution is 1.25, then multiply each solution in the problem by 4 to get 5.
Example 1
Calculate the empirical formula for a compound that contains 1.82g of K, 5.93g of I and 2.24g of O.
| K | I | O | |
|---|---|---|---|
| mass | 1.82 | 5.93 | 2.24 |
| RAM | 39.1 | 126.9 | 16 |
| mass/RAM | 0.0465 | 0.0467 | 0.14 |
| divide by smallest number | 1 | 1 | 3 |
This would give an empirical formula of KIO3.
Example 2
3.51 g of hydrated zinc sulfate were heated and 1.97 g of anhydrous zinc sulfate were obtained. Use these data to calculate the value of the integer x in ZnSO4.xH2O.
| ZnSO4 | H2O | |
|---|---|---|
| mass | 1.97 | 1.54 |
| RMM | 161.5 | 18 |
| mass/RMM | 0.0122 | 0.085 |
| divide by smallest number | 1 | 7 |
This would give the molecular formula of hydrated zinc sulfate as ZnSO4.7H2O.