Define relative atomic mass (Ar) and relative formula mass (Mr). How are they used?

Define relative atomic mass (Ar) and relative formula mass (Mr). How are they used?

• A. Ar is the average mass of an atom relative to 1 gram; Mr is the sum of Ar values in a formula.
• B. Ar is the average mass of an atom relative to 1/12 of carbon-12; Mr is the sum of Ar values in a formula and is used to calculate moles and masses.
• C. Ar is the total number of protons in an atom; Mr is the total number of neutrons.
• D. Ar and Mr are both measured in grams.

Answer: (B)

Relative Atomic Mass, Ar, is the average mass of an atom of an element compared with 1/12 of a carbon-12 atom. It’s a dimensionless comparison, but we use it to work out practical masses by summing Ar values for a formula. Relative Formula Mass, Mr, is the sum of the Ar values of all atoms in a formula unit. This gives the molar mass of that substance—its mass per mole—when the Ar values are combined for the whole formula. These concepts let you convert between mass and moles. The number of moles is the mass divided by Mr: n = mass / Mr. So a substance with a larger Mr needs more mass per mole. For example, in water the Ar values (approximately) are 1 for hydrogen and 16 for oxygen, giving Mr ≈ 2×1 + 16 = 18. That means 1 mole of water has a mass of about 18 g. Ar and Mr are used in calculations to determine masses in reactions and the amounts of reactants or products involved.

Relative Atomic Mass, Ar, is the average mass of an atom of an element compared with 1/12 of a carbon-12 atom. It’s a dimensionless comparison, but we use it to work out practical masses by summing Ar values for a formula. Relative Formula Mass, Mr, is the sum of the Ar values of all atoms in a formula unit. This gives the molar mass of that substance—its mass per mole—when the Ar values are combined for the whole formula.

These concepts let you convert between mass and moles. The number of moles is the mass divided by Mr: n = mass / Mr. So a substance with a larger Mr needs more mass per mole. For example, in water the Ar values (approximately) are 1 for hydrogen and 16 for oxygen, giving Mr ≈ 2×1 + 16 = 18. That means 1 mole of water has a mass of about 18 g. Ar and Mr are used in calculations to determine masses in reactions and the amounts of reactants or products involved.