ggdgf

Higher Chemistry

Mole Calculations
Moles and Number

1 mole of any substance contains the same number of atoms/molecules/ions as there are atoms in a 12g sample of carbon -12

12g of carbon-12 contains a constant number of atoms.

This number is known as the Avogadro constant (L) and is equal to 6.02 x 1023.

1 mole of carbon-12 contains 6.02 x 1023 atoms.

Therefore, 1 mole of any substance contains 6.02 x 1023 atoms/molecules/ions.

For example,

1 mole of aluminium contains 6.02 x 1023 Al atoms
1 mole of ammonia contains 6.02 x 1023 NH3 molecules
1 mole of iron ions contains 6.02 x 1023 Fe ions

Atoms

Calculate the number of atoms in 2 moles of silicon.

Number of Atoms = moles x atoms in 1 mole = 2 x 6.02 x 1023 = 1.20 x 1024 atoms

Calculate the number of atoms in:

(a) 3 moles of phosphorus
(b) 0.5 moles of sodium

Answers (a) 1.81 x 1024 atoms (b) 3.01 x 1023 atoms

Molecules

Calculate the number of molecules in 6 moles of methane

Number of Molecules = moles x molecules in 1 mole = 6 x 6.02 x 1023 molecules = 3.61 x 1024 molecules

Calculate the number of molecules in:

(a) 0.37 moles of sulphur dioxide (b) 3.4 moles of hydrogen chloride

Answers (a) 2.23 x 1023 molecules (b) 2.05 x 1024 molecules

Ions

Ionic compounds do not contain atoms or molecules. Instead, we talk about ‘formula units’

The formula unit (f.u.) is the same as the chemical formula

Calculate the number of formula units in 2 moles of magnesium bromide [MgBr2]

Number of Formula Units = moles x f.u. in 1 mole = 2 x 6.02 x 1023 f.u. = 1.20 x 1024 f.u.

Calculate the number of ions in 0.25 moles of potassium sulphate [K2SO4]

1 formula unit = 3 ions (K + K + SO4)
6.02 x 1023 formula units = 3 x 6.02 x 1023 of ions

Number of ions = moles x ions in 1 mole = 0.25 x 3 x 6.02 x 1023 = 4.52 x 1023 ions

Calculate the number of:

(a) copper ions in 0.4