Question

Solve problem:

Calculate the number of moles of magnesium oxide, MgO in

1. 80 g, and
2. 10 g of the compound.

(Average atomic masses of Mg = 24 and O = 16)

Answer 1

Given: i. Mass of MgO = 80 g
ii. Mass of MgO = 10 g

To find: Number of moles of MgO

Formulae: Number of moles (n) = `"Mass of a substance"/"Molar mass of a substance"`

Calculation: i. Molecular mass of MgO = (1 × Average atomic mass of Mg) + (1 × Average atomic mass of O)
= (1 × 24 u) + (1 × 16 u)
= 40 u
∴ Molar mass of MgO = 40 g mol⁻¹
Mass of MgO = 80 g
Number of moles (n) = `"Mass of a substance"/"Molar mass of a substance"`

= `(80 "g")/(40 "g mol"^-1)`

= 2 mol

ii. Mass of MgO = 10 g, Molar mass of MgO = 40 g mol^–1

Number of moles (n) = `"Mass of a substance"/"Molar mass of a substance"`

= `(10 "g")/(40 "g mol"^-1)`

= 0.25 mol

1. The number of moles in 80 g of magnesium oxide, MgO = 2 mol
2. The number of moles in 10 g of magnesium oxide, MgO = 0.25 mol