All About Moles

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Masses of Atoms

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?

What's Wrong?

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: ___________________).

How Many Atoms Are There in One Gram of Carbon?

What would we need to know?



?

The Mole

A mole is a unit for particles.

One mole is 6.02 * 1023 particles.

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?

Molar Mass

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?

Formula Mass

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

Beware of Gases

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.

Answer: 31.5 moles

Polyatomic Ions (Review?)

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

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.

Determining Formula Mass from Quantities

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.

Set 17 from booklet

Calculating Formula from Masses of Constituents

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:

___________________.

Limiting Reagants/Reactants

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'.

Rocket Fuel

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?

Hydrated Salts

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)

Concentration of Solutions

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?

Percentage Yield

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.

A Hydrogen Balloon

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%.

Old IGCSE Questions

From handout.