Percentages of Elements
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses in the numbers shown in the formula.
The percentage of an element in a compound can be calculated from the relative mass of the element in the compound and the relative formula mass of the compound. (AQA GCSE Chemistry Unit 2)
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Percentages of Elements in Compounds

Composition of   ()

1. The relative formula mass of g  
2. The % composition by element (to 1 dp):

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