Chemistry - Moles and Mass

What is the Ar?

Relative atomic mass - the mean mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom

What is the Mr?

relative formula mass

How do you calculate percentage mass?

Ar of element / Mr of substance

Do you use the small numbers or balancing when calculating Mr?

Use the small numbers but not the balancing numbers

When should you use the balancing numbers for calculating the Mr

When calculating atom economy

What is a mole?

The amount of any substance containing as many entities as there are carbon atoms in exactly 12g of the carbon-12 isotope

Avagadro’s constant

• The number of atoms in 12g of the carbon-12 isotope

• The number of particles in one mole of any substance

• 6.02 × 10²³

One mole of any substance =

Mr (in grams)

Number of molecules =

moles x avagadro’s constant

Number of atoms =

moles x avagadros’s constant x number of atoms in formula

Mass =

Mol x Mr

How do you calculate the theoretical mass of a product?

1. convert mass to moles

2. find the mole ratio

3. find the moles of product

4. convert moles to mass

How do you calculate the limiting reactant?

1. Find the moles of both reactants using moles = mass / Mr

2. find the mole ratio

3. Calculate how much of one substance is needed to react with the other

What is an example of wording for the limiting reactant?

0.1 mol Na needs 0.05 mol F₂ to fully react. There is more F₂ present than this therefore F₂ is in excess and Na is limiting.

How do you calculate the theoretical mass of a product when one of the reactants is limiting?

1. Workout limiting reactant

2. Find mole ratio of limiting reactant to product

3. Find moles of product using mole ratio

4. Find mass of product

How do you form a balanced equation?

1. Calculate moles

2. Find the mol ratio

3. Turn the mol ratio into balancing numbers

Percentage yield =

actual yield / theoretical yield x 100

Why would the percentage yield be less than 100%?

• the reaction may be reversible therefore the products might turn back into the reactants

• unwanted reactions may happen / other products may form instead of what was expected

• some of the product may have been left in the apparatus

• reactants may not be pure

Atom economy =

Mr of desired product / Mr of all products x 100

What are the formulas for silver, zinc, sulphate, carbonate, nitrate, ammonium and hydroxide?

Ag⁺, Zn²⁺, SO₄^2-, CO₃^2-, NO₃^-, NH₄⁺, OH^-

Practice Question - How many moles of oxygen atoms are there in 500g of CaCO₃?

Mr of CaCO³ = 100

Moles = Mass / Mr —→ 500 / 100 = 5

Each molecule of CaCO³ contains 3 oxygen atoms

5 × 3 = 15

Answer: 15 moles of oxygen atoms