Relative Formula Mass (Mr) Flashcards

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What is relative formula mass (Mr)?

It is the sum of the relative atomic masses (Ar) of all the atoms in a formula.

Ar is relative atomic mass. You take each element's Ar from the periodic table and add them.

No. Mr has no units because it is a relative number.

Ar(C)=12 and Ar(O)=16, so Mr = 12 + (16 x 2) = 44.

Ar(Mg)=24 and Ar(Cl)=35.5, so Mr = 24 + (35.5 x 2) = 95.0.

Ar(Ca)=40, Ar(O)=16, Ar(H)=1; Mr = 40 + (16 + 1) x 2 = 74.

Ar(Na)=23, Ar(C)=12, Ar(O)=16; Mr = (23 x 2) + 12 + (16 x 3) = 106.

Ar(O)=16, so Mr = 2 x 16 = 32 because oxygen is diatomic (two atoms).

No. Ignore big numbers in front. They show how many molecules, not the Mr of one formula unit.

Use the relative atomic mass (Ar) values given on your data sheet, usually the larger number with decimals.

Some Ar values are decimals due to isotopes (e.g., Cl = 35.5), so the total Mr can be a decimal.