Moles - 4th

Question 1

what is relative formula mass?

Answer 1

• the mass of a formula unit of a substance

Question 2

how is RFM calculated?

Answer 2

• by adding the relative atomic masses (RAMs) of the atoms or ions.

Question 3

calculate the RFM for H20.

Answer 3

RFM = (2*1) + 16 = 18

Question 4

calculate the RFM for Ba(OH)2.

Answer 4

RFM = 137 + (2*(16+1)) = 171

Question 5

calculate the RFM for CuSO4 dot 5H20.

Answer 5

RFM = 63.5 + 32 + (416) + (518) = 249.5

Question 6

what is percentage by mass?

Answer 6

• shows the mass of a substance that comes from a particular element.

Question 7

how is percentage by mass calculated?

Answer 7

• from the formula for the substance, using the RFM

Question 8

calculate the percentage of C in C3H8.

Answer 8

%C = (C3/C3H8) * 100
= ((312) / ((312)+8) )* 100
= 81.8%

Question 9

calculate the percentage of N in NH4NO3.

Answer 9

%N = ((N+N)/NH4NO3) * 100
= ((214) / 80) 100
= 35%

Question 10

what is a mole?

Answer 10

• the unit of amount of substance in chemistry
• it represents how many particles of a substance there are, taking into account that particles of different substances have different masses

Question 11

how is the amount of substance in moles calculated?

Answer 11

• from its mass and RFM
• moles = mass (g) / RFM

Question 12

calculate the amount in moles in 11g of CO2.

Answer 12

= mass / RFM
= 11/44 = 0.25 mol

Question 13

calculate the mass of 2.5 moles of H2O.

Answer 13

= moles * RFM
= 2.5 * 18
= 45g

Question 14

what does the empirical formula of a substance contain?

Answer 14

• the simplest mole ratio of its elements

Question 15

how can empirical formula be calculated?

Answer 15

• from the masses (or percentages) of each element
  1. convert the masses (or percentages) into moles
  2. cancel down the mole ratio
  3. write down the simplest mole ratio into a formula

Question 16

calculate the empirical formula of a substance containing 6.2g P and 4.8g O .

Answer 16

• draw a ‘mass, moles, ratio’ table and stick the numbers in:
  P O
  mass (g) 6.2 4.8
  moles 0.2 0.3
  ratio 2 3
• whole number ratio = 2:3
• P203

Question 17

calculate the empirical formula of a substance containing 83.7% C and 16.3% H.

Answer 17

• use the percentages as masses (as if we had 100g of the substance
• draw a ‘mass, moles, ratio’ table and stick the numbers in:
  C H
  mass (g) 83.7 16.3
  moles 6.975 16.3
  ratio 3 7
• whole number ratio = 3:7
• C3H7

Question 18

what leads to errors in empirical formula calculations?

Answer 18

• rounding moles is v.dangerous and can lead to errors
• you can get away with rounding moles to 2dp, but never round to 1dp or whole numbers

Question 19

what does the molecular formula of a substance contain?

Answer 19

• the actual numbers of atoms in a molecule - like empirical formula, but uncancelled

Question 20

how can the molecular formula be calculated?

Answer 20

• from the empirical formula and RFM
• this works by finding the uncancelled multiple of the empirical formula with the correct RFM

Question 21

calculate the molecular formula if the empirical formula is CH2 and the RFM is 70.

Answer 21

• RFM of CH2 = 14
• multiple = 70/14 = 5
• molecular formula = 5 * CH2 = C5H10

Question 22

what do hydrated salt crystals contain?

Answer 22

• water trapped inside them in a fixed mole ratio
• this water is called water of crystallisation and it is written into a formula like so:
  CuSO4 “DOT” 5H2O
  salt “dot” water of crystallisation

Question 23

what happens to hydrated salt crystals when they are heated?

Answer 23

they lose their water as vapour
- e.g.
CuSO4”dot”5H2O -> CuSO4 + 5H2O

Question 24

how is water of crystallisation calculated?

Answer 24

• from the masses of salt and water produced on heating

Question 25

calculate 'x' if 16.7g of MgCl2"dot"xH20 crystals decreased in mass to 9.5 on heating.

Answer 25

1. calculate the mass of water lost - 16.7g - 9.5g = 7.2g 2. draw a 'mass, moles, ratio' table and stick the numbers in. be careful to use the final mass of solid for the salt, not the original mass of crystals MgCl2 H2O mass (g) 9.5 7.2 moles 0.1 0.4 ratio 1 4 - whole number ratio = 2:3 - x = 4 - MgCl2 "dot" 4H2O

Question 26

what term do chemists use when there isn't enough substance to do a reaction?

Answer 26

- the substance is limiting

Question 27

what term do chemists use when there is more than enough substance to do a reaction?

Answer 27

- the substance is in excess

Question 28

what happens when we mix 0.9 moles of Zn with 1.1 moles of CuO?

Answer 28

- Zn + CuO -> ZnO + Cu - the equation tells us the moles of both should be the same, so it must be that: Zn is limiting, because there isn't enough of it and CuO is in excess, because there's too much of it.

Question 29

what does the limiting substance determine?

Answer 29

- how many moles are actually used and made

Question 30

how can you work out which substance is in limiting/excess?

Answer 30

- because there are multiple substances in a reaction, each with a mass and potentially different balancing numbers in the reaction equation, it can be difficult to work out which is in excess - pick one at random, then calculate how much of the other would be needed to react with it, and then compare that to how much of the other there actually is.

Question 31

what happens if we mix 10g of Mg and 10g of HCl?

Answer 31

- Mg + 2HCl -> MgCl2 + H2 - start with Mg and calculate how much HCl would be needed to react with that amount of Mg, and then compare that to how much HCl there actually is. - 1Mg : 2HCl - 0.416 moles : 0.833... - mass of HCl - 30.4g - 10g of Mg would need to react with 30.4g of HCl, but we only have 10g of HCl! - HCl is limiting - there isn't enough - Mg is in excess

Question 32

how is percentage yield of a reaction calculated?

Answer 32

% yield = (experimental (from experiment) / theoretical (from calc)) * 100

Question 33

how is experimental mass and theoretical mass found?

Answer 33

- EM - measured in an experiment (or given to you in question) - TM - calculated based on the mass of a reactant

Question 34

calculate the percentage yield if 10g of Al produces 0.96g of H2 in an experiment.

Answer 34

1. find/invent a balanced equation for the reaction : 2Al + 6HCl -> 2AlCl3 + 3H2 2. calculate the theoretical mass of H2 that 10g of Al should have produced - 2Al : 3H2 mole ratio - 2 : 3 actual moles - 0.37 : 0.55 - mass of H2 = 1.11g - % yield = (0.96/1.11) * 100 = 86.4%