## Monday, 29 December 2014

### 3.3 Number of Moles and Mass

Number of moles and mass
Definition : Molar mass: The mass of a substance that contains one mole of the substance.
 ► Molar mass of substance = mass of 1 mole of substance ► Molar mass of substance = mass of substances for $6.02×{10}^{23}$ particles.
■ Relationship between the number of moles, the mass molar and the mass of a substance.

Molar mass of a substance is equivalent to their relative atomic mass or relative molecular mass or relative formula mass written in the unit of gram.

For example:
 ○ The relative atomic mass of nitrogen, N = 14, thus the molar mass of nitrogen atom is 14g. ○ The relative molecular mass of nitrogen, ${\text{N}}_{2}$ = 14 x 2 = 28, thus the molar mass of a molecule = 28g.
■ Summary of relationship between the number of moles, the mass molar and the mass of a substance.
Worked-example 3.3(a)
Calculate the number of particle in
 (a) Calculate the mass of 0.5 mole chlorine ${\text{Cl}}_{2}$ [RAM : Cl = 35.5]
 Solution The mass of 0.5 mole Chlorine ${\text{Cl}}_{2}$ Number of moles $\begin{array}{c}\text{=}\frac{\text{Mass of Chlorine}}{\text{Relative molecular mass}}\end{array}$ $\begin{array}{c}\text{0.5 =}\frac{\text{Mass of Chlorine}}{2.0×35.5}\end{array}$ Mass of Chlorine = $0.5×71$ Mass of Chlorine = 35.5g
 (b) Find the number of moles in 32g Copper, Cu. [RAM : Cu = 64]
 Solution Number of moles $\begin{array}{c}\text{=}\frac{\text{Mass of Copper}}{\text{Relative molecular mass}}\end{array}$ Number of moles $\begin{array}{c}\text{=}\frac{32}{64}\end{array}$ Number of moles = 0.5mol

Worked-example 3.3(b)
Calculate the number of particles in 20g Methane, ${\text{CH}}_{4}$ , [RAM: H = 1, C = 12]
 Solution Number of moles $\begin{array}{c}\text{=}\frac{\text{Mass of Methane}}{\text{Relative molecular mass}}\end{array}$ Number of moles$\begin{array}{c}\text{=}\frac{\text{20}}{12+\left(4×1\right)}\end{array}$ Number of moles = 1.25mol Number of particles = Number of moles $×$ Avogrado constant Number of particles = $1.25×6.02×{10}^{23}$ Number of particles = $7.75×{10}^{23}$