Molecular And Empirical Formula

Question 1

What is empirical formula

Answer 1

A formula in its lowest ratio

Question 2

How to find molecular from the empirical formula and Mr

Answer 2

1) find the Mr of the empirical formula
2) see how many time the Mr of the empirical formula goes into the Mr of the compound (divide them)
3) multiply all the numbers in the empirical formula with the number you just found
This is the molecular formula

Question 3

How to find empirical formula from masses or percentages
(A compound is found to contain 50.05% sulfur and 49.95% oxygen by weight, what is the empirical formula of the compound)

Answer 3

1) assume 100g of the compound is present. Change the percentages given into grams
Sulfur 50.05% —> 50.05g
Oxygen 49.95% —> 49.95g

2) convert the masses into moles
(moles = mass/Mr)
Sulfur 50.05g/32 = 1.564 moles
Oxygen 49.95g/16 = 3.1222

3) divide both values by the lower number, then round to the nearest whole number to form a ratio
Sulfur 1.564/1.564 = 1
Oxygen 3.122/1.564 = 1.996
Rounded = 2
Now we have the ratio 1:2

4) use the ratio to write the empirical formula for the compound
SO2