2. Masses of Atoms
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Imagine we were to measure the mass of atoms (very
difficult, but not impossible!).Which would have greater mass:
one carbon atom or one hydrogen atom?
How many hydrogen atoms would we need to have the same
mass as one carbon-12 atom?
3. What's Wrong?
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Not all hydrogen atoms contain only one proton.
Some contain a neutron, some contain two
neutrons. These are called ______________ of
Hydrogen.
For this we will assume we are using the most
common isotopes of each element. This is
common practice at Grade 10 (incl IGCSE).
It emphasises conceptual understanding over long
calculations with lots of decimals.
Please use the IGCSE Periodic Table provided on
the back of the booklets.
(Exception: ___________________).
4. How Many Atoms Are There in One
Gram of Carbon?
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What would we need to know?
?
5. The Mole
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A mole is a unit for particles.
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One mole is 6.02 * 1023 particles.
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6.02 * 1023 is called Avogradro's Number, named
after Amedeo Avogadro (1776-1856, Italian).
A mole is a much more practical measurement of
matter than atoms.
1. How many moles in 120.4 * 1023 molecules of
carbon dioxide?
2. How many molecules are there in 3 moles of
sulphuric acid?
3. How many molecules in 5 moles of chlorine gas?
6. Molar Mass
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One mole is the number of atoms in 12 grams of
carbon-12. Carbon-12 has 12 grams / mole. This is
called the relative atomic mass.
This rule works (approximately) for every element:
the mass number is the number of grams per mole.
1. How many moles in 2g of sodium?
2. How many moles in 1kg of iron?
3. How many atoms in 24 grams of carbon?
4. How many atoms are there in 32 grams of
copper?
5. How many atoms are there in 48 grams of
magnesium?
7. Formula Mass
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Since molecules are made of atoms, the
relative formula mass is the sum of the masses
of the atoms it is made of.
Eg Calculate the formula mass of Copper
Sulphate.
Answer: 160 g/mol
Determine the number of moles in 80 grams of
copper sulphate.
Copper = 64 g/mol
Sulphur = 32 g/mol
Oxygen = 16 g/mol
Answer: 0.5 moles
8. Beware of Gases
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Gases form covalent compounds (they share
electrons).
Most gases are found in nature in pairs.
Eg. Calculate the number of moles in 1kg of
Oxygen.
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Answer: 31.5 moles
9. Polyatomic Ions (Review?)
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Polyatomic ions contain more than one atom.
They must be treated as a whole ion, and written in
brackets if there is more than one.
eg. Magnesium Hydroxide
Copper Sulphate
Aluminium Carbonate
Sodium Sulphate
Sodium Hydrogen Carbonate
10. Set 15 - Mole – Mass II
Page 3 in handout.
Don't forget Oxygen comes in
pairs.
9. and 10. are complicated
ions.
Set 16: Mass - Mole
Page 4 in handout.
Don't forget Hydrogen comes
in pairs.
11. Determining Formula Mass from
Quantities
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The formula mass can be calculated from
known quantities and numbers of moles.
Eg One can of coke contains about 40 grams of
sugar. This is about 0.22 moles of sugar
(glucose). Calculate the molecular mass of
glucose.
Note: this number will be wrong if high fructose
corn syrup or other sugars are used instead.
13. Calculating Formula from Masses
of Constituents
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A sample of sodium oxide is found to contain 10
grams of sodium and 3.48 grams of oxygen.
Determine the formula for sodium oxide.
a) calculate the number of moles of
10g Sodium: _______________
3.48g Oxygen (as single atoms): ________
b) Determine the formula of sodium oxide:
___________________.
14. Limiting Reagants/Reactants
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It can be important in reactions to have the
same amount of reactants so that neither is
wasted.
Since reactions occur at the level of atoms, it is
important to have the same number of moles of
each.
The reactant which will run out is called the
'limiting reagant'. The other reactant is said to
be 'in excess'.
15. Rocket Fuel
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Rocket fuel is often made from hydrogen and oxygen.
Calculate the mass of oxygen required to react with
1kg of hydrogen.
a) Write a balanced equation for the combustion of
hydrogen:
b) Calculate the number of moles of hydrogen in 1kg.
c) How many moles of Oxygen will be required to react
with the hydrogen from b)?
d) How many kg of Oxygen will be required for
To check your answer:
e) Calculate the molecular mass of water.
f) How many moles of water molecules will be
produced?
g) Calculate the mass of water produced. What does
this show?
16. Hydrated Salts
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Some salts have hydrated and non-hydrated
forms.
Anhydrous copper sulphate (white powder)
reacts with water to form hydrous copper
sulphate (blue crystals).
CuSO4(s) + 5H2O(l) --> CuSO4.5H2O(s)
17. Concentration of Solutions
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Concentrations of solutions (such as acids and
bases) are often measured in moles per litre.
1 Litre = 1 dm3.
Eg How many moles in 100mL of
Hcl at 2 mol/L?
18. Percentage Yield
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In chemistry, a yield is the amount of a product
obtained in a reaction.
The fractional/percentage yield is the ratio of
actual yield to the theoretical maximum yield
(based on the reactants used).
In the 'real world' it's not always possible to
measure the amounts of reactants perfectly,
and reactions do not always finish, so most
'real-world' reactions have low percentage
yields.
19. A Hydrogen Balloon
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A balloon contains 100g of Hydrogen gas. It is
burned in excess oxygen, in a closed system,
and the water vapour collected. 700g of water
vapour are collected.
a) Calculate the theoretical yield (from the last
question).
b) Calculate the percentage yield.
c) Suggest a reason why the percentage yield
could not reach 100%.
