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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