Formula, equations and amount of substance

Relative atomic mass (A_r)

The average mass of an atom of an element on a scale where an atom of carbon-12 is 12

Relative isotopic mass

The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12

Relative molecular/formula mass (M_r)

The average mass of a molecule or formula unit on a scale where an atom of carbon-12 is 12

Empirical formula

The smallest whole number ratio of atoms in a compound

Molecular formula

The actual numbers of atoms in a molecule

Molar mass

The mass of 1 mole of a substance (g mol^-1). It is equal to the A_r/M_r of the substance.

Mole equation

n = mass / molar mass (A_r or M_r)

Concentration equation

Concentration (mol dm^-3) = n (mol) / volume (dm³)

Percentage composition equation

(Total mass of element in compound / total mass of compound) x 100

Percentage yield equation

(Actual yield / theoretical yield) x 100

Why is actual yield always less than theoretical yield

• Reactants don’t fully react

• Side reactions occur

• Some chemicals may get lost during transfer between apparatus