Unit 9 - Calculations Involving Masses

Question 1

What is an empirical formula?

Answer 1

The simplest whole number ratio of atoms or ions of each element in a substance

Question 2

What is a molecular formula?

Answer 2

The actual number of atoms of each element in one molecule

Question 3

Give an example where the empirical formula is different to the molecular formula

Answer 3

Ethene - molecular formula = C2H4, empirical formula = CH2

Question 4

Give an example where the empirical formula is the same as the molecular formula

Answer 4

Water - molecular formula = H20, empirical formula = H20

Question 5

What is the relative formula mass? (Mr)

Answer 5

The sum of the relative atomic masses of all he atoms or ions in the formula

Question 6

How do you work out the empirical formula using the amount of each substance?

Answer 6

Write each element in its own column
Divide the % or g by the RAM
Divide each of these answers by the smallest answer.
This is the atomic ratio for the empirical formula

Question 7

How do you find the molecular formula from the empirical formula?

Answer 7

Find the empirical formula mass (total mass)
Divide the relative formula mass by the empirical mass (should be given)
The answer is multiply the empirical formula by to get the molecular formula.

Question 8

What is the law of conservation of mass

Answer 8

The mass of the solution is equal to the mass of the solvent and the mass of the solute.

Question 9

What is the concentration

Answer 9

The amount of solute dissolved in a stated volume of solution

Question 10

What is 1 dm^3 equal to

Answer 10

1 litre or 1000cm^3

Question 11

How can you calculate the the concentration of a solution

Answer 11

Concentration = mass of solute in g / volume of solution in dm^3

Question 12

What are the units for concentration

Answer 12

gdm^-3

Question 13

How do you convert from cm^3 to dm^3

Answer 13

Divide by 1000

Question 14

What is a closed system?

Answer 14

When no new substances are added or removed

Question 15

Give an example of a closed system

Answer 15

Lead nitrate + potassium iodide -> Lead iodide + potassium nitrate

Question 16

What is a non-enclosed system?

Answer 16

When the mass of the products appear to change from the reactants. E.g from a gas

Question 17

Give an example of a non-enclosed reaction

Answer 17

Copper carbonate -> copper oxide + carbon dioxide

Question 18

How do you calculate the mass of a substance needed to make a stated amount of product

Answer 18

Write the balanced equation
Find the Mr of the substances needed (excluding the balanced numbers)
Multiple each of the Mr by their balancing numbers
Since we know the product mass, divide both answers by the previous answer for the product (get one g of product but must do the same for reactant)
Multiply both answers by the stated mass given in the question

Question 19

What is the Avogadro number?

Answer 19

6.02 x 10^23

Question 20

What does the Avogadro number mean?

Answer 20

The amount of particles in 1 mole of a substance

Question 21

What is a mole?

Answer 21

An amount of a substance which contains the Avogadro number. The relative atomic mass is the amount of g in 1 mole of a substance

Question 22

Give an example of an element, the mass of 1 mole, and the amount of atoms it contains

Answer 22

Magnesium, 1 mole = 24g, it contains 6.02 x 10^23 atoms.

Question 23

What is the equation for calculating the smith of moles in a substance?

Answer 23

Moles = g/Ar or Mr

Question 24

What is a limiting reactant?

Answer 24

Usually one of the reactants is added in excess and not completely used up, the limiting reactant is the reactant that is not in excess

Question 25

How do you work out the limiting reactant?

Answer 25

Work out how many moles of 1 substance there is in the reaction. (g/Mr or Ar)
Multiply this by any balancing numbers for both substances. This will give the amount of moles for the other substance needed to react with it.
Calculate how many moles there are in the other substance
This will show if there is less or more than needed to react with the first substance.

Question 26

How do you work out the mass of a product formed using the masses of the reactants

Answer 26

The equation should show that the amount of moles in the product will equal an amount of moles in one of the reactants.
Calculate the moles of the reactant using it’s mass and Ar.
Multiply this by the Mr of the product formed.

Question 27

What is the stoichiometry of a reaction?

Answer 27

The ratio of the moles of each substance.

Question 28

How do you work out the balanced equation using the masses of the substances?

Answer 28

Calculate the moles of each substance
Divide each by the smaller answer
Multiply both by a factor so that they are both whole numbers.
Add the formula and balance