Calculating the Empirical Formula and Molecular Formula
The empirical formula is the simplest formula and only tells you the ratio of the various atoms. Suppose 2.4g of magnesium combined with 1.6g of oxygen, you can use a table to work out the empirical formula. (Mg = 24 O = 16)
|
|
C | H |
|
Percentage |
87.5 | 14.3 |
|
Combining Masses |
87.5 | 14.3 |
|
Number of moles |
85.7/12 |
14.3/1 |
| = | 7.14 |
14.3 |
|
Ratio of Moles |
1:2 |
|
|
Empirical Formula |
CH2 |
|
| Mg | O | |
| Combining Masses | 2.4 | 1.6 |
| Number of moles | 2.4/24 | 1.6/16 |
| = | 0.10 | 0.10 |
| Ratio of Moles | 1:1 | |
| Empirical Formula | MgO | |
What about with percentage figures?
Suppose you had a compound containing 85.7% C, 14.3% H and you were asked to calculate the empirical formula. Firstly, you assume that 100% = 100g! (C = 12 H = 1)
However, you know that CH2 does not exist. Remember this is only the ratio. To find the molecular formulae, you need to know the relative formula mass of the compound. Suppose it was 56g for the above question.
Firstly, find out the RFM of CH2 = 12 + 2 = 14g
Find out how many times 14 goes into 56, so 56/14 = 4 times
Which means the molecular formula is C4H8!