ย 
โ† PREVIOUS TOPIC
LOADING FULL SCREEN

Formulae for OCR A-level Chemistry

Formulae

This page covers the following topics:

1. Empirical and molecular formulae

The empirical formula represents the simplest whole number ratio of elements in a compound. The molecular formula denotes all the atoms of each element present in a compound. To get a molecular formulae from diagrams, count the number of atoms of each kind.

To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden โ€œ1โ€ (when there is just a single atom of some element in a compound) and divide all of the subscripts in the formula by this greatest common factor.

To determine the empirical formula from a percentage composition, take a 100 g of a material, in which case mass in grams of each element will be the same as the percentage. Find the number of moles for each element by dividing by the mass number of an atom, and divide each by the smallest number of moles you got. You may multiply the ratio you get from the divisions by additional numbers to get integers.

To convert an empirical formula to a molecular formula, find how many times the relative formula mass of the compound is greater than the relative formula mass of the empirical formula. Then multiply the numbers of atoms in the empirical formula by that number to get the molecular formula.

Empirical and molecular formulae

1

Do NOโ‚‚ and Nโ‚‚Oโ‚„ have the same empirical formula?

To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden โ€œ1โ€ (when there is just a single atom of some element in a compound) and divide all of the subscripts in the formula by this greatest common factor.

For NOโ‚‚ the highest common factor of 1 and 2 is 1. Dividing each of the subscripts by 1 gives 1 and 2. The โ€œ1โ€ is excluded from the empirical formula to give NOโ‚‚.

For Nโ‚‚Oโ‚„ the highest common factor of 2 and 4 is 2. Dividing each of the subscripts by 2 gives 1 and 2. The โ€œ1โ€ is excluded from the empirical formula to give NOโ‚‚.

yes

Do NOโ‚‚ and Nโ‚‚Oโ‚„ have the same empirical formula?

2

What is the empirical formula of glucose if it has a molecular formula of Cโ‚†Hโ‚โ‚‚Oโ‚†?

To get an empirical formula from the molecular formula, find the highest common factor of all the subscripts in molecular formula, including the hidden โ€œ1โ€ (when there is just a single atom of some element in a compound) and divide all of the subscripts in the formula by this greatest common factor.

The highest common factor of 6, 12 and 6 is 6. Dividing each of the subscripts by 6 gives 1, 2 and 1. The โ€œ1โ€s are excluded from the empirical formula to give CHโ‚‚O.

CHโ‚‚O

What is the empirical formula of glucose if it has a molecular formula of Cโ‚†Hโ‚โ‚‚Oโ‚†?

3

A compound has an empirical formula CH. Find the molecular formula of this compound if it has a relative formula mass of 78 g/mol.

To convert an empirical formula to a molecular formula, find how many times the relative formula mass of the compound is greater than the relative formula mass of the empirical formula. Then multiply the numbers of atoms in the empirical formula by that number to get the molecular formula.

M(CH) = 12 + 1 = 13 g/mol
M(compound) รท M(CH) = 78 รท 13 = 6
number of carbon atoms in the compound = 1 ร— 6 = 6
number of hydrogen atoms in the compound = 1 ร— 6 = 6

Cโ‚†Hโ‚†

A compound has an empirical formula CH. Find the molecular formula of this compound if it has a relative formula mass of 78 g/mol.

4

A compound has the following percentage composition: 3.06% hydrogen, 31.63% phosphorus and 65.31% oxygen. Determine the empirical formula of the compound.

To determine the empirical formula from a percentage composition, take a 100 g of a material, in which case mass of each element will be the same as the percentage. Find the number of moles for each element by dividing by the mass number of an atom, and divide each by the smallest number of moles you got. You may multiply the ratio you get from the divisions by additional numbers to get integers.

m(H) = 3.06 g
m(P) = 31.63 g
m(O) = 65.31 g

n(H) = 3.06 รท 1 = 3.06 mol
n(P) = 31.63 รท 31 = 1.02 mol (smallest)
n(O) = 65.31 รท 16 = 4.08 mol

n(H) : n(P) : n(O) = 3.06 : 1.02 : 4.08
n(H) : n(P) : n(O) = 3.06/1.02 : 1.02/1.02 : 4.08/1.02
n(H) : n(P) : n(O) = 3 : 1 : 4

Hโ‚ƒPOโ‚„

A compound has the following percentage composition: 3.06% hydrogen, 31.63% phosphorus and 65.31% oxygen. Determine the empirical formula of the compound.

5

Determine the empirical formula of the compound represented in the image.

The compound presented in the diagram has 12 calcium and 12 oxygen ions. To get an empirical formula from the molecular formula, find the highest common factor of all the numbers of atoms in the compound and divide all of them by this greatest common factor.

The highest common factor of 12 and 12 is 12. Dividing each of the numbers by 12 gives 1 and 1. The โ€œ1โ€s are excluded from the empirical formula to give CaO.

CaO

Determine the empirical formula of the compound represented in the image.

End of page

ย