5.11 - Empirical formulae

Question 1

What does the term ‘empirical’ indicate?

Answer 1

That some information has been found by experiment

Question 2

What does an empirical formula show?

Answer 2

The lowest whole number ratio of atoms of each element in a compound

Question 3

Give an example of an experimental method to determine the empirical formula of CuO:

Answer 3

• Place a known mass of copper oxide in a tube
• Heat it in a stream of hydrogen gas
• Hydrogen atoms react with oxygen atoms in the copper oxide, forming steam
• Solid gradually changes colour from black to orange-brown
• Excess gas is burnt off at the end of the tube for safety
• Remove and weigh the copper once it has cooled

Question 4

What are the steps for calculating the empirical formula of a compound?

Answer 4

• Divide the mass/percentage mass of each element in the compound by its atomic mass
• Divide the new numbers by the smallest new number
• This will give an obvious ratio
• The ratio is used to write the formula

Question 5

How many significant figures should be used in the calculations, as a minimum?

Answer 5

2, but preferably 3

Question 6

Calculate the empirical formula of the oxide of copper using the following data:

• Mass of copper oxide = 4.28g
• Mass of copper = 3.43g

Answer 6

• Mass of oxygen = 4.28g - 3.43g = 0.85g

Cu -> 3.43g/63.5 = 0.0540
O -> 0.85g/16 = 0.0531

Ratio = 1:1

Empirical Formula = CuO

Question 7

If the subscripts in a formula will reduce, is the formula the empirical formula?

Answer 7

No, because it is not the smallest whole number ratio of elements in the compound, as it can be divided further