Topic 3 - Equations, Calculations & Electrolysis

Question 1

relative atomic mass (Ar)

Answer 1

average mass of all isotopes of that element

Question 2

relative formula mass (Mr)

Answer 2

sum of relative atomic masses in a compound

Question 3

how to find percentage mass of an element within a compound

Answer 3

Ar of element / Mr of compound

Question 4

Moles equation

Answer 4

Moles = mass / Mr

Question 5

Avogadro Number

Answer 5

6.02x10^23

Question 6

conservation of mass

Answer 6

in a chemical reaction, no atoms are created or destroyed, the bonds are just changed

Question 7

why might the mass of products be less than the reactants?

Answer 7

• reactants could have spilled
• products (if gas) could have floated off

Question 8

why might the mass of products be greater than the reactants?

Answer 8

• gas from air reacting with reactants

Question 9

Calculating mass in reactions

Answer 9

1. write out balanced equation
2. find moles of known substance (moles = mass/mr)
3. find moles of unknown substance (using molar ratio)
4. find mass of unknown substance (mass = moles x mr)

Question 10

Volume of gas at RTP equations

Answer 10

Volume = moles x 24 dm^3

Volume = moles x 24,000 cm^3

cm -> dm = /1000

Question 11

Concentration equation

Answer 11

concentration = mass / volume

Question 12

molecular formula

Answer 12

actual number of atoms of each element in a compound or molecule

Question 13

empirical formula

Answer 13

smallest whole number ratio of each element in a compound or molecule

Question 14

Finding empirical formula

Answer 14

Create a table with all elements separated

• identify mass
• find moles (mass/mr)
• divide by smallest number of moles
• find ratio!

Question 15

Finding molecular formula

Answer 15

• Divide unknown compound’s Mr by Mr of empirical formula
• multiply all numbers in formula by the result of division

Question 16

Water of crystallisation

Answer 16

when salt crystals contain water molecules

hydrated -> contain water

anhydrous -> do not contain water

E.g. anhydrous copper sulfate turns blue when water is added

Question 17

How is water of crystallisation shown?

Answer 17

Using a dot followed by the number of water molecules there are.

Question 18

Percentage yield equation

Answer 18

yield obtained / theoretical yield

x100

Question 19

why might the percentage yield be less than 100%

Answer 19

• reactants may not all react
• products may be lost to surroundings

Question 20

electrolysis

Answer 20

when an electric current passes through a substance and causes it to break down

Question 21

positive electrode

Answer 21

anode

where negative ions go

Question 22

negative electrode

Answer 22

cathode

where positive ions go

Question 23

Oxidation

Answer 23

gain of oxygen, loss of electrons

OILRIG

Question 24

Reduction

Answer 24

loss of oxygen, gain of electrons

OILRIG

Question 25

electrolyte

Answer 25

liquid in which electrolysis takes place

Question 26

How to extract metals from their ore when they are LOWER in reactivity than carbon.

Answer 26

displacement reaction with carbon

Question 27

How to extract metals from their ore when they are HIGHER in reactivity than carbon

Answer 27

electrolysis

Question 28

What happens to OH- in electrolysis

Answer 28

**oxidised ** at the **anode**

Question 29

What happens to H+ in electrolysis

Answer 29

**reduced** at the **cathode**