4:Molecular And Empirical Formulae

Question 1

What is molecular formula

Answer 1

Show the number of atoms of each element in one molecule of a substance

Question 2

What is the empirical formula

Answer 2

Shows the simplest whole number ratio of atoms of each element present in a substance

Question 3

How to calculate the percentage of a mass of an element in a compound

Answer 3

% mass = mass of the element in the compound ➗ total mass of the compound

Question 4

What are the steps to calculate empirical formula

Answer 4

1. Write the mass or %mass of each element
2. Divide each mass by the Ar of that element
3. Divide each of these by the smallest value
4. If the numbers are not whole numbers (or very close to) then multiply them all by 2 or 3 etc till they are all whole numbers

Question 5

How to calculate molecular formula using empirical formula

Answer 5

Divide the Mr of the compound by the Mr of the empirical formula to find the ratio.
Multiply the empirical formula by the ratio

Question 6

How to calculate empirical formula using reaction data

Answer 6

e.g. Compound X is a hydrocarbon.

When it burns in oxygen, all of the carbon from X is converted to CO2 and all of the hydrogen is converted to water.

A sample of X is burned and the products are 8.80g CO2 and 4.50g H2O.

mol CO2 = mass / Mr = 8.80 / 44.0 = 0.200 mol

Each mole of CO2 contains 1 mole C, so mol C in the sample of X = 0.200

mol H2O = mass / Mr = 4.50 / 18.0 = 0.250 mol

Each mol H2O contains 2 mol H, so mol H in the sample of X = 0.250 x 2 = 0.500

Use the mol of each element in the sample to calculate empirical formula normally.

Question 7

How to calculate the ratio of water of crystallisation (x)

Answer 7

1. Calculate the number of moles of the compound after heating.
2. Calculate the mass of water lost.
3. Work out the mol of water lost
4. Divide the mol of water by the mol of the compound to find the ratio (x).

Question 8

How to identify an element in a hydrated compound.

Answer 8

1. Find the mass of water boiled away
2. Work out the mol of water lost
3. Use the mol ratio of water to the compound to work out the mol of the compound.
4. Calculate Mr of compound using the mol and anhydrous mass
5. Take away the Mr of the other element(s) in the compound to identify the element