Formulas

Question 1

What does the empirical formula give?

Answer 1

The smallest whole number ratio of atoms in a compound.

Question 2

What does the molecular formula give?

Answer 2

The actual numbers of atoms in a molecule.

Question 3

What are the steps of working out the molecular formula?

Answer 3

Find the empirical mass.
Divide the molecular mass by the empirical mass (this tells you how many multiples of the empirical formula are in the molecular formula).
Multiply the empirical formula by that number to find the molecular formula.

Question 4

How do you calculate the empirical formula from experimental data?

Answer 4

Divide each mass by its relative atomic mass to find moles.
Simplify the ratio (divide each by the smallest).
Write the formula.

Question 5

How do you calculate the empirical formula from percentage compositions?

Answer 5

1. Assume you’ve got 100g of the compound- you can turn the percentages straight into masses. Then you can work out how many moles of each element are in 100g of the compound.
2. Divide each number of moles by the smallest number of moles you found in step 1. This gives you the ratio of the elements in the compound.
3. Apply the numbers from the ratio to the formula.

Question 6

How do you calculate molecular formula from the empirical formula?

Answer 6

Dividing the molar mass by the mass of the empirical formula tells you what factor you need to scale the empirical formula up by to get the molecular formula.