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Science

Chemical calculations - higher

Empirical formulas

You can use information about reacting masses to calculate the formula of a compound [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ]. Here is an example:

Question

Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

Use the fact that the Ar of sulfur is 32 and the Ar of oxygen is 16.

toggle answer

Answer
  • Find the mass of each element. Conservation of mass tells us that the mass of oxygen = the mass of sulfur oxide - the mass of sulfur.
  • The mass of oxygen reacted = 6.4 - 3.2 = 3.2g
  • So we have 3.2g of sulfur and 3.2g of oxygen.
  • Now divide the mass of each element by its Ar value.
  • sulfur: 3.2 ÷ 32 = 0.1
  • oxygen: 3.2 ÷ 16 = 0.2
  • Finally, find the ratio of the elements.
  • You can do this by dividing the results by the smallest of the numbers to give you the number of atoms of each element in the compound.
  • In this case the smallest value is 0.1, so divide both results by that.
  • S = 0.1 ÷ 0.1 = 1
  • O = 0.2 ÷ 0.1 = 2
  • (If one of the numbers ends in 0.5, multiply all the numbers by 2 - this is because you cannot have half-atoms in a compound.)
  • So the ratio of sulfur to oxygen is 1:2

The number of atoms tells you that the formula for sulfur oxide is SO2

Here is the calculation again in tabular form to help you remember the steps:

Steps to calculation the formula of a compound

stepactionSO
1find masses3.23.2
2look up given Ar values3216
3divide masses by Ar0.10.2
4find the ratio12

Result: the formula for the oxide = SO2 

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