Relative Formula Mass Flashcards

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What is meant by relative formula mass, Mr, of a compound?

It is the total of the relative atomic masses of all the atoms in its formula.

Which numbers from the periodic table do you use to work out relative formula mass?

You use the relative atomic masses, the larger numbers that may have decimal points.

Does relative formula mass have any units?

No. Relative formula mass, Mr, has no units.

Describe the steps to calculate the relative formula mass of a compound.

Find the relative atomic mass of each element, multiply by the number of its atoms, then add all the values.

How do you calculate the relative formula mass of water, H2O?

Hydrogen: Ar = 1 and there are 2 atoms, so 2 × 1 = 2. Oxygen: Ar = 16 and there is 1 atom, so 1 × 16 = 16. Mr = 2 + 16 = 18.

What is the relative formula mass of magnesium chloride, MgCl2? (Mg = 24, Cl = 35.5)

Mg: 24 × 1 = 24. Cl: 35.5 × 2 = 71. Mr = 24 + 71 = 95.

What is the relative formula mass of calcium carbonate, CaCO3? (Ca = 40, C = 12, O = 16)

Ca: 40 × 1 = 40. C: 12 × 1 = 12. O: 16 × 3 = 48. Mr = 40 + 12 + 48 = 100.

How do you deal with brackets when finding the relative formula mass of Ca(OH)2?

Work out the total for the atoms inside the brackets, then multiply by the number outside. For Ca(OH)2: O = 16 and H = 1, so O + H = 17. Then 17 × 2 = 34. Add Ca: 40. Mr = 40 + 34 = 74.

When do chemists use 'relative formula mass' instead of 'relative molecular mass'?

We use relative formula mass for ionic compounds and giant structures. We use relative molecular mass for simple molecules.

Why is it called relative formula mass?

Because it is based on relative atomic masses, which compare masses to one twelfth of a carbon-12 atom.

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