4S1Monday, 14/2/201110.45 – 11.55 am

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Relationship of Number of Particles and Number of Moles

Number of moles = Number of particles

NA

Number of particles = Number of moles

× NANumber of moles (mol)

Number of

particles

× NA

÷ NA

Molar mass

= The mass of one

mole of a substance

Unit : g mol-1

One mole of any substance contains 6.02x1023 particles.

Therefore, the molar mass of a substance contains 6.02x1023

particles of the substance

RAM Molar mass

Value refer Periodic

Table

Same as RAM

Unit No unit g mol-1

RAM vs Molar mass

Element Relative mass

Molar mass

Helium 4 4 g mol-1

Sodium 23 23 g mol-1

Water, H2O 2(1) + 16 = 18 18 g mol-1

Ammonia, NH3 14 + 3(1) = 17 17 g mol-1

Sodium chloride, NaCl

23 + 35.5 = 58.5

58.5 g mol-1

Lithium oxide, Li2O

2(7) + 16 = 30

30 g mol-1

Element Relative mass Molar mass

FeSO4

K2O

Al(OH)3

NaNO3

MgCO3

56 + 32 + 4(16) = 152

152 g mol-1

2(39) + 16 = 94 94 g mol-1

27 + 3(16 + 1) = 78 78 g mol-1

23 + 14 + 3(16) = 85

84 g mol-124 + 12 + 3(16) = 84

85 g mol-1

Relationship of Mass and Number of Moles

Number of moles = Mass Molar mass

Mass = Number of moles x Molar mass

Number of moles (mol)

Mass (g)

× Molar mass

(g mol-1)

÷ Molar mass

(g mol-1)

Example 1Find the number of moles of 22.1 g of AgNO3

Molar mass of AgNO3

= [108 + 14 + 3(16)] g mol-1 = 170 g mol-1

Number of moles of AgNO3

= mass of AgNO3

molar mass of AgNO3 = 22.1 g 170 g mol-1

= 0.13 mol

Example 2Find the number of moles of 4.55 g of B2O3

Molar mass of B2O3

= [2(11) + 3(16)] g mol-1 = 70 g mol-1

Number of moles of B2O3

= mass of B2O3

molar mass of B2O3

= 4.55 g 70 g mol-1

= 0.065 mol

Example 3Find the mass of 0.86 mol of MgCl2

Molar mass of MgCl2

= [24 + 2(35.5)] g mol-1 = 95 g mol-1

Mass of MgCl2

= number of moles of MgCl2 × Molar mass of MgCl2

= 0.86 mol × 95 g mol-1 = 81.7 g

Example 4Find the mass of 0.13 mol of C2H4

Molar mass of C2H4

= [2(12) + 4(1)] g mol-1 = 28 g mol-1

Mass of C2H4

= number of moles of C2H4 × Molar mass of C2H4

= 0.13 mol × 28 g mol-1 = 3.64 g

Exercise 1Find the number of moles of 23 g of NO2

Molar mass of NO2

= [14 + 2(16)] g mol-1 = 46 g mol-1

Number of moles of NO2

= mass of NO2

molar mass of NO2

= 23 g 46 g mol-1

= 0.5 mol

Exercise 2Find the mass of 0.1 mol of SO3

Molar mass of SO3

= [32 + 3(16)] g mol-1 = 80 g mol-1

Mass of SO3

= number of moles of SO3 × Molar mass of SO3

= 0.1 mol × 80 g mol-1 = 8.00 g