Topic 1.2: Formulas

1.2.1 Define the terms relative atomic mass (Ar) and relative molecular mass (Mr)

Relative Atomic Mass: The mass of one mole of atoms of an element is Relative Atomic Mass (Ar), expressed in grams.

But masses of all elements are in the range of 10^-24 to 10^-22 grams and these numbers are way too inconvenient to use practically. The mass was then recorded relative to the common element carbon. Carbon-12 is the most abundant isotope of carbon, but because Carbon-13 and Carbon-14 also exist, the average relative atomic mass (Ar) is greater than 12.

Relative Molecular Mass: The Relative Molecular Mass (Mr) is calculated by adding the relative atoic masses of the atoms making up the molecule.

1.2.2 Calculate the mass of one mole of a species from its formula

Mass = Molar Mass x Moles


1.2.3 Solve problems involving the relationship between the amount of substances in moles, mass and molar mass.

Simple tips, remember the formula is interchangeable

Mass = Molar Mass x Moles

Molar Mass = Mass / Moles

Moles = Mass / Molar Mass

Also Moles is constant in an chemical equation, except when there is a Mole Ratio.

1.2.4 Distinguish between the terms empirical formula and molecular formula.

Empirical Formula is the simplest ratio known to a chemical formula

Molecular Formula is a multiple of the empirical formula, can only be determined once the relative molecular mass is known. It also show the actual number of atoms in the molecule

1.2.5 Determine the empirical formula from the percentage composition or from other experimental data.

Determine the elements in the molecule.

Fine the Molar Mass of these individual elements.

Divide the percentage composition of each element to their respective Molar Mass to determine moles

Find the ratio between the moles to create the Empirical Formula

1.2.6 Determine the molecular formula when given both the empirical formula and the experimental data.

Experimental data should include the Molar Mass of the Molecular Formula.

Find Molar Mass for Empirical Formula

Divide the Molar Mass of Molecular Formula by the Molar Mass for Empirical Formula.

Multiple all elements by the same number produced to receive the Molecular Formula.