ChemSolutions. They all contain the same number of particles (atoms, molecules or ions) BUT HOW MANY...

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