Topic 5 - Formulae, Equations and Amounts of Substance

Question 1

what is a mole (mol)?

Answer 1

the mole is the amount of substance in grams that has the same number o particles as there atoms in 12 grams of carbon-12

Question 2

what is the relative atomic mass (Ar)?

Answer 2

the Ar is the average mass of one atom compared to 1/12th of the mass of one atom of carbon-12

Question 3

what is the molar mass of a substance?

Answer 3

the mass in grams of 1 mole of a substance and is given the unit of g mole-1

Question 4

what is the relative molecular mass (Mr)?

Answer 4

the Mr is the average of a molecule compared to 1/12th of the mass of an atom of carbon-12

Question 5

what is avogadros constant (L)?

Answer 5

6.02 x10^23 mol-1

Question 6

what is the equation linking number of moles, number of particles and avogadros constant?

Answer 6

number of moles = number of particles / avogadros constant

Question 7

what is molar mass equal to?

Answer 7

molecular mass

Question 8

what is the equation linking number of moles, mass (g) and molar mass?

Answer 8

number of moles = mass (g) / molar mass

Question 9

what is the equation linking number of moles, mass (g) and Mr?

Answer 9

number of moles = mass (g) / Mr

Question 10

what’s the equation linking gas volume (dm3), amount of moles?

Answer 10

gas volume (dm3) = amount of moles x 24

Question 11

what’s the equation linking concentration, amount of moles and volume and what are the units?

Answer 11

concentration (mol dm-3) = amount of moles / volume (dm3)

Question 12

how do you get from cm3 to dm3

Answer 12

divide by 1000

Question 13

how do you get from cm3 to m3?

Answer 13

divide by 1 000 000

Question 14

how do you get from dm3 to m3

Answer 14

divide by 1000

Question 15

what is the empirical formula?

Answer 15

the simplest whole number ratio of atoms of each element present in a compound

Question 16

how do I find the mass of a substance using volume and concentration include units?

Answer 16

mass of substance = concentration (g dm-3) x volume (dm3)

Question 17

What is meant by ‘water of crystallisation’?

Answer 17

Water molecules that are chemically bound within the crystal structure of an ionic compound

Question 18

What term is used for an ionic compound that contains water of crystallisation?

Answer 18

hydrated

Question 19

what term is used for an ionic compound that does not contain water of crystallisation?

Answer 19

anhydrous

Question 20

How can hydrated and anhydrous forms of the same compound differ?

Answer 20

they often differ in physical appearance, such as colour

Question 21

what is the colour of anhydrous copper (II) sulphate?

Answer 21

white

Question 22

what is the colour of hydrated copper (II) sulfate

Answer 22

blue

Question 23

how are hydrated compounds denoted?

Answer 23

with a dot separating the salt from the water molecules
eg CuSO4*5H2O

Question 24

What is the purpose of heating hydrated calcium sulfate in a crucible?

Answer 24

To remove the water of crystallisation and measure the mass loss during thermal decomposition.

Question 25

Why must the crucible be dry before use?

Answer 25

To avoid additional mass loss from water evaporating, which would give inaccurate results.

Question 26

Why is a lid used during the experiment and why must it be loose fitting?

Answer 26

To prevent loss of solid from the crucible while still allowing gases to escape loose fitting to allow water vapour (gas) to escape during heating.

Question 27

Why should large amounts of hydrated compound (e.g., 50g) be avoided?

Answer 27

Why should large amounts of hydrated compound (e.g., 50g) be avoided

Question 28

Why is it necessary to heat and reweigh until constant mass is achieved?

Answer 28

To ensure that the reaction is complete and all water has been removed.

Question 29

Why should very small amounts of sample (e.g., 0.100g) be avoided?

Answer 29

Errors in weighing are proportionally too large, reducing accuracy.

Question 30

How can this method be used to measure mass gain?

Answer 30

By heating magnesium in oxygen, where it reacts to form magnesium oxide and gains mass.

Question 31

how do you calculate the percentage composition of an element while using the empirical formula?

Answer 31

percentage composition of element X = (total mass of the element in the compound / total mass of the compound) x100

Question 32

what's the formula connecting density with mass and volume

Answer 32

density = mass/volume