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 6.02×1023 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, N2 = 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 Cl2 [RAM : Cl = 35.5]

Solution
The mass of 0.5 mole Chlorine Cl2
Number of moles =Mass of Chlorine Relative molecular mass
0.5 =Mass of Chlorine 2.0×35.5
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 =Mass of Copper Relative molecular mass
Number of moles =32 64
Number of moles = 0.5mol

Worked-example 3.3(b)
Calculate the number of particles in 20g Methane, CH4 , [RAM: H = 1, C = 12]

Solution
Number of moles =Mass of Methane Relative molecular mass
Number of moles =20 12+(4×1)
Number of moles = 1.25mol
Number of particles = Number of moles × Avogrado constant
Number of particles = 1.25×6.02×1023
Number of particles = 7.75×1023


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1 comment:

  1. Very Useful information Thank you!!
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