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Relative Atomic Mass (Ar)

The relative atomic mass of any atom is the number of times the mass of one atom of an element is greater than 1/12 of the mass of one carbon-12 atom.

Relative Atomic Mass (Ar) = Mass of one atom of the element Mass of 1/12 of an atom of carbon - 12

Element Relative atomic mass, ArHydrogenSodiumCopperChlorine

Why are some Ar values not whole numbers?

________________________________________________________________________* Recall: Isotopes are atoms of the same element with the same number of protons but different number of neutrons.

Example: A sample of chlorine is made up of 75% of chlorine-35 atoms and 25% of chlorine-37 atoms.

Relative Molecular Mass (Mr)

The relative molecular mass ( Mr ) of an element or compound is the mass of a molecule, compared to 1/12 the mass of one atom of carbon-12.

Relative Molecular Mass (Mr) = Mass of one molecule of the element Mass of 1/12 of an atom of carbon - 12

Find out the Mr of the following covalent compounds.Compound Chemical Formula Relative molecular mass, Mr

Methane

Ammonia

Sulphur dioxide

Ethanol C2H5OH

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Secondary 3 Express Science (Chemistry)Chapter 7: Relative masses of Atoms and MoleculesNotes

Relative Molecular Mass (M r)The ‘relative molecular mass’ of an ionic compound is more accurately known as relative formula mass.

Find out the Mr of the following ionic compounds.Compound Chemical Formula Relative formula mass, MrMagnesium sulphate

Calcium nitrate

Ammonium phosphate

Copper (II) sulphate crystals

CuSO4. 5H2O

Practice Questions:1. An unknown element X has 3 naturally occurring isotopes.

Calculate the relative atomic mass of the unknown element X.

2. Calculate the Mr of(a) a molecule of chlorine gas: ________________________________________

(b) a molecule of DDT (C14H9Cl5): _____________________________________

(c) aluminium phosphate: ______________________________________________

(d) FeSO4.7H2O: _____________________________________________________

3. Ascorbic acid, also known as vitamin C, is given the formula C6H8Ox. The relative molecular mass of ascorbic acid is 176. What is the value of the unknown, x?

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Isotopes Isotopic AbundanceX – 36 5%X - 38 5%X - 40 90%

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