The mole

The mole is a common unit used to represent the quantity of a chemical substance and is the amount of that substance that contains 6.2023×10²³ particles (Avogadro’s constant).
Number of moles =mass of substance/R.F.M

 

Calculating masses

Na₂SiO₃

R.F.M= (R.A.M Sodium x 2) + (R.A.M Silicon) + (R.A.M Oxygen x 2)
R.F.M= (23×2) + 28+ (16×3) = 36+12+48 =96

Al₂S3

R.F.M= (27×2) + (32×3) = 54+96=150

Percentage composition

Calculate the percentage of carbon in sodium carbonate

Percentage = (R.A.M) /R.F.M × 100

 

Na₂CO3

R.F.M of Na₂CO3 = (23×2) +12+ (16×3) =106

Percentage of C =(12/106) x 100= 11.32%

Percentage of O = (R.A.M × 3(number of oxygen’s))/R.F.M = (48/106) x100 = 45.28%

 

Please note that 1 mole of a substance = R.F.M in grams of a substance

Moles Example

Calculate the number of moles of sodium chloride in 58.5g.

Number of moles = mass of substance/R.F.M = 58.5g/ 58.5 = 1

Calculate the mass of 0.12moles of sodium

1 mole of Na = 23g

0.12 mole of Na=? g

Mass of substance = R.F.M × number of moles = 23 x0.12 = 2.76g Na

1 mole of any substance contains 6.023×10²³ molecules, atoms or ions.

Example 1; 1 mole of Sodium Chloride contains 6.023×10²³ ions.

Example 2; 1 mole of CH₄ contains 6.023×10²³ molecules.

Example 3; 1 mole of Li contains 6.023×10²³atoms.