ChemSolutions

They all contain the same number of particles (atoms, molecules or ions)

BUT HOW MANY PARTICLES .......?

602,000,000,000,000,000,000,000

approximately !!!

and usually written as

6.02 x 1023

and called

Avogadro's Constant (L)

6.02 x 1023 particles = 1 mole

CALCULATIONS IN CHEMISTRY

• A mole is an amount of substance. • ONE MOLE of ANYTHING contains an Avogadro

Number of particles (atoms, molecules or ions)

• 6.02 x 1023 particles = 1 mole of substance

To find the number of moles we need to know the relative formula mass (Mr) of a substance:

Relative formula mass = Relative atomic masses (Ar)

Eg. What is the Mr of water (H2O)

Ar (H) = 1 Ar(O) = 16

Mr (H2O) = 1+1+16

= 18

MOLAR MASS (H2O) = 18g/mole

Find the Mr of the following :

Cl2

CH4

NaCl

CaCO3

Find the Mr of the following :

Cl2 35.5 +35.5 = 70.0

CH4 12 + (4x1) = 16

NaCl 23 + 35.5 = 58.5

CaCO3 40 + 12 + (3x16) = 100

Moles and Mass Conversion

Formula is: MOLES = MASS___________ (n) RELATIVE FORMULA MASS

Example 1

How many moles of magnesium atoms are in 6g of magnesium metal?

m = 6g Ar = 24

n = m Ar

= 6/24

= 0.25 moles

Example 2

How many moles of substance are present in 10g of calcium carbonate?

n = m Mr

= 10/100

= 0.1 moles

m = 10g Mr = (40 + 12 + 48) = 100

Example 3

How many moles of substance are present in 585g of sodium chloride?

m = 585g Mr = (23 + 58.5) = 58.5

n = m Mr

= 585/58.5

= 10 moles

Example 4

What is the mass of 0.01 moles of copper sulphate (CuSO4)?

m = ? g Mr = (64 + 32 + (4x16)) = 160

m= n x Mr

= 0.01 x 160

= 1.6 g

Example 5

8.8g of a gas contains 0.2 moles of gas molecules. What is the Relative Formula Mass (Mr) of the gas?

What could the gas be?

m = 8.8 g Mr = ? n = 0.2

Mr= m n = 8.8/0.2

= 44Possibly CO2

Molar mass from molecular formulae

1. Calculate the molar masses of the following:

(a) P4O10

(b) Al2(S04)3

(c) (NH4)2CO3

(d) K4Fe(CN)6

(e) NH4Fe(SO4)2.12H2O

Mass to mole conversions

2. How many moles of substance are present in the following?

(a) 0.250 g of calcium carbonate

(b) 5.30 g of anhydrous sodium carbonate

(c) 5.72 g sodium carbonate-10- water crystals

a) 284 g/mole b) 342 g/molec) 96 g/moled) 368 g/molee) 482 g/mole

a) 0.0025b) 0.05c) 0.02

ANSWERS