Topic 9 - Calculations Involving Masses

Question 1

1.43 - How do you calculate the relative formula mass of a compound?

Answer 1

Relative formula mass is Mr.
It is found by adding all the relative atomic masses ( RAM ) of that compound.
CO2 - C = 12 and O = 16
12 + 16*2 = 44

Question 2

1.44 - How do you calculate empirical formulae?

Answer 2

Empirical formulae - simplest whole number ratio of the number of atoms or ions of each element in a compound.
To calculate the formula from reacting masses:
Find the moles using moles = mass/mr
Work out ratio of moles and the simplest whole numbers possible.
Write the moles as subscripts in the formulae.

Question 3

1.45 - How do you find the molecular formula?

Answer 3

Molecular formulae - the actual number of atoms of each element in one molecule.
Find the relative molecular mass ( Mr ).
Mr/Empirical to find multiplier.
Multiply the subscripts in formulae by this number.

Question 4

1.46 - Describe an experiment to determine the empirical formula

Answer 4

Magnesium oxide is made by heating magnesium ribbon in a limited oxygen supply.
If the reactants ( Oxygen and magnesium ) are measured the empirical formula for magnesium oxide can be calculated.

Question 5

1.47 - Explain the law of conservation of mass

Answer 5

Law of conservation of mass - When a solute is dissolved in a solvent, the mass of the solution = solute + solvent.
A precipitation reaction in a closed system would mean no substances are added or removed, as the number of atoms aren’t changed, the mass isn’t changed, this is the law of conservation of mass.
A reaction in a non-enclosed system could lead to the appearing to decrease as gas could escape which decreases the amount of atoms in the reaction.
However, some solids could seem to gain mass when heated in a non-enclosed system, this is due to the gases reacting with the surrounding air which explains the apparent gain. Although the mass of the products and reactants in all systems are the same.

Question 6

1.48 - How do you calculate the masses of reactants and products in a reaction?

Answer 6

Calculate the mass of chlorine to make 53.4g of aluminium chloride

2Al + 3Cl2 –> 2AlCl3
Mr of Cl2 - 71
Mr of AlCl3 - 133.5
mass = moles * mr
mass = 2133.5 = 267g
mass = 371 = 213g
Al : 267/267 : Cl2 : 213/267
Al : 1g : Cl2 : 0.798g
Al : 53.4g : Cl2 - 42.6g

Question 7

1.49 - How do you calculate concentration?

Answer 7

cm3/1000 = dm3
concentration = mass/volume

Question 8

1.50 & 1.51 - How do you calculate number of moles and particles of a substance?

Answer 8

number of moles = mass/mr
moles = particles/avogadros constant

Question 9

1.52 - What controls the mass of product formed in a reaction?

Answer 9

In a reaction, one of the reactants are usually added in excess. The amount of reactant that isn’t in excess or is used completely in the reaction determines the amount of product formed. This is the limiting reactant.

Question 10

1.53 - Deduce the stoichiometry of a reaction

Answer 10

If you know the mass, you can calculate the moles, the moles of each substance is the stoichiometry.e

Question 11

1.53 - Deduce the stoichiometry of a reaction

Answer 11

If you know the mass, you can calculate the moles, the moles of each substance is the stoichiometry.e