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The empirical formula of a chemical compound is the simplest positive integer ratio of atoms present in a compound. An example is the empirical formula of sulfur monoxide, or SO, would simply be SO. Similarly the empirical formula of disulfur dioxide, S₂O₂ is also SO because a 2:2 ratio can be reduced to a 1:1 ratio
Determining a Empirical Formula
Determining a Empirical Formula
One of the most useful skills in chemistry is being able to determine the identity of an unknown chemical. We see this a lot on TV crime dramas. Through experimentation scientists can determine the relative mass of each element in a chemical sample. From these relative percentages we can determine the chemical formula of the compound.
Steps:
Steps:
Assume that there is 100g of the chemical sample, so that the percentages can be taken for grams.
Divide the mass of each element by the molar mass. This will give you moles.
The empirical formula is the ratio of each element by using the moles. However, we must have whole numbers in the final formula, so we will divide each amount of moles by the element with the least.
In some cases dividing by the least still leaves us with one or two decimals numbers instead of whole numbers. In this case if the decimal ends in".5" we double all of the moles to get whole numbers. If it ends in ".33" we tripple each moles to get whole number ratios
Example:
Example:
The percentage composition of an acid is found to be 19.4% C, 3.25% H, and 77.39% O. Determine the empirical formula of the acid.
Change % to g
19.4% becomes 19.4g of C
3.25% becomes 3.25g of H
77.39% becomes 77.39g of O.
Divide by molar mass
19.4g ÷ 12.01g/mol = 1.62 mol C
3.25g ÷ 1.01 g/mol = 3.22 mol H
77.39g ÷ 16.0 g/mol = 4.83 mol O
Divide by the least number of moles
1.62 ÷ 1.62 ≈ 1 C
3.22 ÷ 1.62 ≈ 2 H
4.83 ÷ 1.62 ≈ 3 O
Final formula H2CO3
Practice
Practice
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