Amount of substance

Question 1

What are the 3 equations used to calculate moles?

Answer 1

moles = mass / molar mass ( n = m / M )

moles = concentration x volume ( n = c x V ) - volume in dm^3

moles = volume / molar gas volume ( n = V / Vm )

Question 2

What is the molar gas volume?

Answer 2

24dm^3mol^-1

Question 3

What conditions are needed to be able to use the equation n = V / Vm?

Answer 3

Room temperature and pressure ( RTP )

Question 4

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

Answer 4

divide by 1000

Question 5

What are the conditions at RTP?

Answer 5

101kPa

298K

Question 6

Define empirical formula

Answer 6

The simplest whole number ratio of atoms of each element in a compund

Question 7

Define anhydrous

Answer 7

Containing no water molecules

Question 8

Define water of crystallisation

Answer 8

Water molecules that are bonded into a crystalline structure

Question 9

Define hydrated

Answer 9

A crystalline compound containing water molecules

Question 10

What happens when blue crystals of copper ( II ) sulfate are heated?

Answer 10

The bonds holding the water within the crystal are broken and the water is driven off, leaving behind white anhydrous copper ( II ) sulfate

Question 11

Describe the practical to find the formula of a hydrated salt and state 2 assumptions made

Answer 11

1 ) Weigh an empty crucible
2 ) Add the hydrated salt into the weighed crucible then weigh the crucible and hydrated salt together
3 ) Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about one minute. Then heat it strongly for a further three minutes
4 ) Leave the crucible to cool then weigh the crucible and anhydrous salt
5) Calculate the amount ( in mol ) of the anhydrous salt
6 ) Calculate the amount ( in mol ) of water
7 ) Find the smallest whole number ratio
8 ) Write down the formula of the hydrated salt

Assumptions:

• We assume all of the water has been lost ( solution is to heat to constant mass as this suggests all the water has evaporated )
• We assume there was been no further decomposition ( many salts decompose further when heated )

Question 12

What is the ideal gas equation?

Answer 12

pV = nRT

pressure (Pa) x volume ( m^3) = moles x ideal gas constant x temperature ( K )

Question 13

What assumptions are made about the molecules making up an ideal gas?

Answer 13

• random motion
• elastic collisions
• negligible size
• no intermolecular forces

Question 14

How do you get from degrees to kelvin?

Answer 14

+273

Question 15

Define stoichiometry

Answer 15

The ratio of the amount, in moles, of each substance in a chemical equation ( ratio of balancing numbers )

Question 16

Why is theoretical yield difficult to achieve?

Answer 16

• the reaction may have not gone to completion
• other reactions ( side reactions ) may have taken place alongside the main reaction
• purification of the product may result in loss of some of the product

Question 17

What is the equation used to calculate percentage yield?

Answer 17

percentage yield = ( actual yield / theoretical yield ) x 100

Question 18

What is a limiting reagant?

Answer 18

The reactant that is not in excess, which will be used up first and stop the reaction

Question 19

What is atom economy?

Answer 19

A measure of how well atoms atoms have been utilised

Question 20

What is the equation to work out atom economy?

Answer 20

atom economy = ( sum of molar masses of desired products / sum of molar masses of all products ) x 100

Question 21

What are the benefits of a reaction having a high atom economy?

Answer 21

• produce a large proportion of desired products and few unwanted waste products
• makes industrial processes more efficient, preserves raw materials and reduces waste
• high atom economy is important for sustainability