AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Empirical formula is the simplest whole number ratio of atoms of each…
Full specification wording
Empirical and molecular formula: Empirical formula is the simplest whole number ratio of atoms of each element in a compound.
The empirical formula gives the ratio of atoms, reduced as far as it will go, so ethanoic acid and glucose both reduce to CH₂O even though their molecules differ enormously.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 4
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The empirical formula gives the ratio of atoms, reduced as far as it will go, so ethanoic acid and glucose both reduce to CH₂O even though their molecules differ enormously. The standard route from composition is: treat percentages as masses in 100 g, divide each by the relative atomic mass of that element to get a relative amount, then divide every result by the smallest of them. If a value such as 1.5 appears, multiply the whole set by 2 to reach whole numbers, since the ratio must be whole. Where a question names only some elements and says the rest is another, that missing percentage must be found first by subtracting from 100.
What earns the marks
- Finding a missing percentage by subtracting the stated percentages from 100.
- Dividing each percentage or mass by the relative atomic mass of that element.
- Dividing all the relative amounts by the smallest value.
- The whole-number ratio written as a formula, multiplying up where a half or a third remains.
Common mistakes and how to put them right
Dividing by the atomic number instead of the relative atomic mass.
Rounding a ratio of 1.5 down to 1 rather than doubling the whole set.
Omitting the element described as the rest of the sample.
Giving the molecular formula when the empirical formula was asked for.
Exam tips
- Set the work out as a table with one column per element; the process marks are awarded row by row.
- Keep three significant figures in the relative amounts so a genuine 1.5 is not lost to rounding.
- Check the finished formula by confirming no common factor is left in the subscripts.
Try the lesson
The start of the Empirical and molecular formula: Empirical formula is the simplest whole number ratio of atoms of each element in a compound. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement defines the empirical formula of a compound?
Listen
The empirical formula gives the ratio of atoms, reduced as far as it will go, so ethanoic acid and glucose both reduce to CH₂O even though their molecules differ enormously. The standard route from composition is: treat percentages as masses in 100 g, divide each by the relative atomic mass of that element to get a relative amount, then divide every result by the smallest of them. If a value such as 1.5 appears, multiply the whole set by 2 to reach whole numbers, since the ratio must be whole. Where a question names only some elements and says the rest is another, that missing percentage must be found first by subtracting from 100. A strong answer earns marks for these points. finding a missing percentage by subtracting the stated percentages from 100. dividing each percentage or mass by the relative atomic mass of that element. dividing all the relative amounts by the smallest value. the whole-number ratio written as a formula, multiplying up where a half or a third remains. Exam tip: Set the work out as a table with one column per element; the process marks are awarded row by row.
Key words
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empirical formula
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dividing by the smallest
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scaling up fractions
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See every definition
- empirical formula
- The simplest whole number ratio of atoms of each element in a compound.
- dividing by the smallest
- All the relative amounts are divided by the smallest value to reach the ratio.
- scaling up fractions
- Where a value such as 1.5 appears, the whole set is multiplied by 2 to reach whole numbers, since the ratio must be whole.
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