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 $6.02\times {10}^{23}$ 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 $\begin{array}{c}\text{=}\frac{\text{Mass of Chlorine}}{\text{Relative molecular mass}}\end{array}$ $\begin{array}{c}\text{0.5 =}\frac{\text{Mass of Chlorine}}{2.0\times 35.5}\end{array}$ Mass of Chlorine = $0.5\times 71$ 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$\begin{array}{c}\text{=}\frac{\text{20}}{12+\mathrm{(4}\times \mathrm{1)}}\end{array}$ Number of moles = 1.25mol Number of particles = Number of moles $\times$ Avogrado constant Number of particles = $1.25\times 6.02\times {10}^{23}$ Number of particles = $7.75\times {10}^{23}$

← 3.2 Number of Moles and the Number of Particles

3.4 Number of Moles and Its Molar Volume →

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