We can use what we know about the Law of Definite Proportions and the Law of Conservation of Mass to figure out the formula for a compound.
When we derive the chemical formula from observations made during an experiment.That means from properties that we can measure directly.
Let’s say we have 100 grams of magnesium oxide. What is the chemical formula?
What we really want to know is the ratio of one element to another.
Example:
60% magnesium and 40 % oxygen = 60 grams magnesium and 40 grams oxygen
- Convert mass to moles. (Remember we must always go through moleville when dealing with ratios!)
Moles of Magnesium = 60 g
24.31 g/mol
= 2.47 moles of Magnesium
Moles of Oxygen = 40 g
16.00 g/mol = 2.50 moles of Oxygen
This will tell us how many moles of each we have in the whole sample (100g).
- Determine the ratio of Mg : O.
2.47: 2.5 is approximately 1:1 (Hint: think 2.47/2.5)
- The empirical formula of our compound, magnesium oxide must be MgO.
Problem: If we use empirical information and find that the ratio is 1:2, we cannot tell the difference between 1:2 and 2:4. The empirical formula tells us the simplest ratio of elements.
If we want to know the actual number of atoms of each element or the molar ratio (Remember 1 mole = 6.02 x 1023 atoms), we must use our knowledge of moles.
Finding the Molecular Formula from Percentage Composition
Butane: %C = 82.5% %H = 17.5% Molar Mass of Butane = 58 g/mol
- Find the mass of each element in one mole of the compound.
Rather than working with individual atoms, we will work with 1 mole to make calculations easier and 58 grams total mass. Remember we are dealing with ratios so it doesn’t matter what numbers we start with.
Mass of C = 0.825 x 58 g = 47.8 grams C
Mass of H = 0.175 x 58 = 10.2 grams H
2. Convert mass to moles.
Moles of C = 47.8 g = 3.98 mol
12.01 g/mol
Moles of H = 10.0 mol
In one step:
82.5 x 1 mole = 3.98 mol
100 12.01g
3. Use the mole ratio to find the subscripts of your chemical formula.
3.98 moles C : 10 moles H
The molecular formula is C4H10.