Calculations

Question 1

How to work out empirical formula?

Answer 1

Divide masses by relative atomic mass (to find moles)
Divide each number by the smallest one
These numbers show the ratio required

Question 2

What is a mole?

Answer 2

The amount of substance that contains 6.022x10(23) particles

Question 3

What is Avogadro’s number?

Answer 3

6.022x10(23) (the same as the number of particles in each mole of carbon 12)

Question 4

One mole of any substance is equal to?

Answer 4

It’s relative atomic mass turned into grams

Question 5

What are moles measured in?

Answer 5

g/mol

Question 6

How to work out number of moles?

Answer 6

Mass divide by formula mass

Question 7

How to work out mass?

Answer 7

Moles multiplied by formula mass

Question 8

How to work out formula mass?

Answer 8

Mass divided by moles

Question 9

What is percentage composition?

Answer 9

The percentage by mass of each element present in a compound

Question 10

How to work out percentage composition?

Answer 10

Multiple relative atomic mass by number of atoms
Divide by molecular mass of compound
Multiple by 100

Question 11

What is molecular formula?

Answer 11

The actual number of atoms of each element in a molecule.

Question 12

What is empirical formula?

Answer 12

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

Question 13

How to work out molecular formula?

Answer 13

Find empirical formula
Divide actual mass by empirical formula mass

Question 14

What is a solution?

Answer 14

A mixture of solute and solvent particles

Question 15

Concentration definition?

Answer 15

Amount of solute in a given amount of solution

Question 16

Molarity meaning?

Answer 16

An expression of concentration 1M = 1 mole of substance in 1dm(3) of its solution

Question 17

How to work out concentration?

Answer 17

Number of moles divided by volume

Question 18

Molar gas volume definition?

Answer 18

The volume per mole of gas molecules at a stated temperature and pressure

Question 19

What is the volume of one mole of any gas at room temperature and pressure?

Answer 19

24dm(3)

Question 20

How to convert cm(3) to dm(3)?

Answer 20

Divide by 1000

Question 21

How to calculate the volume of gas (dm3)?

Answer 21

Number of moles of gas x 24

Question 22

How to calculate the number of moles of a gas?

Answer 22

Volume of gas (dm3)
——————————
24

Question 23

Percentage yield definition?

Answer 23

Shows how much product is obtained compared to the maximum possible mass

Question 24

Atom economy definition?

Answer 24

The percentage of atoms in reactants that form the desired product

Question 25

Theoretical yield meaning?

Answer 25

The maximum amount of product that could be formed

Question 26

Why is it difficult to achieve theoretical yield?

Answer 26

Reaction may not have gone to completion Other side reactions may have taken place Purification of the product may result in some loss of product Some product may be left behind in apparatus Raw materials may not be pure

Question 27

How to calculate percentage yield?

Answer 27

1) find moles of product 2) find ratio of moles of product to reactant Actual yield —————— x 100 Theoretical yield

Question 28

Limiting reagent meaning?

Answer 28

Any reactant that is not in excess. Meaning the reacting will stop as soon as this is used up.

Question 29

How to work out the limiting reagent?

Answer 29

Used the balanced equation to see which reactant is used up quicker

Question 30

How to work out atom economy?

Answer 30

The mass of useful product ————————————x 100 Total mass of all products

Question 31

Why is it useful to maximise percentage yield and atom economy?

Answer 31

Reduce energy use Reduce costs Conserve raw materials Reduce waste Reduce pollution

Question 32

What is a titration used for?

Answer 32

To calculate the concentration of an unknown

Question 33

How to work out concentration from a titration?

Answer 33

1) calculate the volume, concentration and moles of the known substance 2) use the balance equation to work out mole ratio 3) use the mole ratio to work out the moles of the unknown 4) use the moles and volume of the unknown to finally workout the concentration

Question 34

What is volumetic analysis?

Answer 34

Finding the concentration of a solution by measuring the volume of a solution that will react with a known volume of standard solution

Question 35

What is the equation to work out a dilution?

Answer 35

Initial conc. x initial vol. = new conc. x new vol This is then rearranged as required

Question 36

Saturated solution definition?

Answer 36

A chemical solution containing the maximum concentration of a solute dissolved in the solvent at a particular temperature. No more solute will dissolve.

Question 37

Supersaturated solution meaning?

Answer 37

A solution that contains a higher that saturation concentration of solute. Any slight disturbance causes crystallisation of excess solute

Question 38

Solubility definition?

Answer 38

A chemical property referring to the ability for a given substance to dissolve in a solvent

Question 39

Precipitate meaning?

Answer 39

Insoluble solid that forms during a chemical reaction and separates out in solution

Question 40

How to test if chloride ions are present?

Answer 40

First silver nitrate: white precipitate Then dilute ammonia: precipitate disappears Then concentrated ammonia: N/A

Question 41

How to test if bromide ions are present?

Answer 41

First silver nitrate: cream precipitate Then dilute ammonia: precipitate remains Then concentrated ammonia: precipitate disappears

Question 42

How to test if iodide ions present?

Answer 42

First silver nitrate: yellow precipitate Then dilute ammonia: precipitate remains Then concentrated ammonia: precipitate remains

Question 43

How to test for carbonate ions?

Answer 43

Add dilute HCl acid to sample If effervescence observed carbonate is likely Transfer gas to limewater, if cloudy CO2 is present

Question 44

How to test for sulfate ions?

Answer 44

Add barium chloride to sample. If present white barium sulfate precipitate will form