C5 - Percentage composition and empirical formula

Question 1

What does the molecular formula tell you?

Answer 1

The number and type of each atom in a molecule.

Question 2

What is empirical formula?

Answer 2

The empirical formula is the simplest way of writing a whole number ratio of each type of atom inside a molecule.

Question 3

What is the empirical formula of glucose (C6H12O6)?

Answer 3

CH2O

Divide by 6

Question 4

How do you work out the empirical formula?

Answer 4

1) look up the relative atomic mass of each element
2) write down the mass of each element
3) work out how many moles there are of each element
4) choose the smallest element present
5) divide the moles of each element in the compound by the moles of the smallest compound

Question 5

2.45g of sulfuric acid contains 0.05g H, 0.8g S and 1.6g O. Calculate the empirical formula of sulfuric acid.

Answer 5

1) mass of each element
H - 0.05, S - 0.8, O - 1.6
2) relative atomic mass
H - 1, S - 32, 0 - 16
3) 0.05/1 = 0.05
    0.8/32 = 0.025
    1.6/16 = 0.1
4) divide by smallest
= 2,1,4

Question 6

What is the formula for % mass of an element?

Answer 6

% mass = total mass of element in the compound/relative formula mass of compound x 100