How do you calculate relative formula mass (Mr)?
To calculate relative formula mass (Mr), add up the relative atomic masses of every atom in the formula. Count each element's atoms, remembering to multiply out brackets, then multiply by its Ar and add the totals. For example, CO₂ has Mr = 12 + (2 × 16) = 44. Mr has no units.
Formula
Mr = sum of (Ar × number of atoms) for every element
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