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Quantitative Chemistry
Calculations in Chemistry: part 2
Compound Formation
A particular compound always contains the same
elements.
Independent of how you make the compound the
elements are always present in the same proportion
by mass.
These proportions cannot be changed.
Magnesium oxide always contains 60% Mg and 40% O
by mass.
Reacting amounts of
substance
Relative formula masses can also be used to calculate the
amounts of compounds reacted together or produced in
reactions.
If 0.24g of Mg react with 0.16g of O to produce 0.40g of
MgO.......... How much MgO will be produced by burning 12g of
Mg?
0.24g Mg produces 0.40g of MgO
so 1g of Mg produces 0.40/0.24 g of MgO
= 1.67 g of MgO
so 12g of Mg produces 12 x 1.67 g MgO
= 20g of MgO
Important
Calculations of quantities like these are a very
important part of chemistry.
There is a great deal of information stored in the
formulae and the equation.
Activity
Question 3 on page 181
Extension: Read through pg 178 – 179 (compound
formation and chamical formulea.
Answer question 4 on page 181
The Mole
• When carrying out an experiment a
chemist can not weigh out single atoms
or molecules.
• A counting unit was found.
• The standard unit of a substance is the
relative atomic mass in grams.
• This unit is called 1 mole (1 mol)
• Carbon Ar = 12
• 1 mole of carbon is 12g
In a similar way
banks weigh coins
because they know
how much one coin
weighs
Avogadro‘s Constant (L)
1 mole of any substance contains the same number of
atoms.
6.02 x 1023 atoms per mole
6.02 x 1023 coke cans
stacked together
would cover the
surface of the Earth to
a depth of 200 miles.
Calcualtions involving
the Mole
How to calculate the molar mass.
Write the formula for ethanol C2H5OH
Calcualte Mr
= (2x12) + (5 x 1) + 16 + 1
= 46
• The molar mass of ethanol is 46g/mol
Calculations
For any given mass of a substance you can calculate
the moles present.
Mass
Number
Of Moles
Mr
Example
How many moles are there in 60g of Sodium
Hydroxide?
Mr of NaOH = 23 + 16 + 1 = 40
Molar mass of NaOH = 40g /mol
Number of moles = mass / molar mass
= 60 / 40
= 1.5 mols
Activity
Worksheet 6.2
Working out the
Empirical Formula
We can work out the chemical formula from
experimental data.
Magnesium reacts with oxygen to form magnesium
oxide. In an experiment if we know that 0.24g of Mg
reacts with 0.16g of O2 we can determine the
formula.
Mg
O
Mass combined
o.24 g
0.16g
Molar mass
24 g/mol
16 g/mol
Number of moles
0.01 mol
0.01 mol
Simplest ratio
1
1
formula
MgO
Activity: Calculate the empirical formula
for silicon oxide
Mass
Si
O
47g
53g
Molar mass
Number of moles
Simplest ration
Formula
Complete questions on page 185
A Chemical Footbridge: calculating
reacting amounts
What mass of aluminium oxide is produced when 9.2g of
aluminuim metal react with oxygen?
Balanced equation:
4Al + 3O2
2Al2O3
Ratio 4:2
Convert 9.2 g of Al into moles.
Number of moles = mass/ molar mass = 9.2 / 27 = 0.34 mol
Use the ratio from the equation to work out how many
moles of Al2O3 are produced.
4 mol of Al produce 2 mol of Al2O3
So 0.34 mol of Al produce 0.17 mol of Al2O3
A Chemical Footbridge: calculating
reacting amounts …..continued
Work out the mass of Al2O3
Mr = 102
Mass = molar mass x number of moles
= 102 x 0.17
= 17.3 g
Concentrations of
Solutions
Two types of concentration
Mass concentration - g/dm³
Molar concentration - mol / dm³
1dm³ = 1000 cm³ = 10 cm x 10 cm x 10cm = 1 Litre
Concetration = mass of solute
volume of solution
Concentration
A 1 mol /dm³ solution of sodium
chloride contains 58.5g of NaCl (1
mol) dissolved in water and made
up to a final volume of 1 dm³ or 1
litre.
Concentration
calculations
How many moles of suger are there in 500 cm³ of a
3.0 mol/dm³ sugar solution?
Number of moles = concentration x volume
1000
= 3.0/ 1000
= 1.5 mol
x 500
Example
Calculate the concentration of a solution of sodium
hydrioxide , NaOH, that contains 10g of NaOH in a final
volume of 250 cm³ (= 0.25 dm³)
Calculate number of moles
Mr = 23 + 16 + 1 = 40
Number of moles = mass/ Mr = 10/40 = 0.25 mol
Find the concentration
Number of moles = concentration x volume
Concentration = number of moles / volume
= 0.25 / 0.25
= 1 mol /dm³
Activity
Read pages 192 and 193
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