amount of substance [P1] PAPER 1+2

Question 1

mass/mr/moles equation

Answer 1

moles (mol) = mass (g) / Mr

Question 2

what mass of sodium oxide would be obtained from 46g of sodium?

Na + O2 -> Na2O

Answer 2

1. balance equation:
   4Na + O2 -> 2Na2O
2. work out moles of Na using moles = mass/mr:
   moles of Na = 46/23 = 2 mol
3. use equation to work out molar ratio of Na:Na2O:
   4:2 = 2:1
4. use this ratio to calculate the moles of the mass of the compound you are wanting to work out:
   2/2 × 1 = 1 mol
5. use moles × mr = mass to calculate the unknown mass:
   1 × (23×2 + 16) = 52g of sodium oxide

Question 3

A sample of hydrated nicken sulfate (NiSO4•xH2O) with a mass of 2.287g was heated to remove all water of crystallisation. The solid remaining had a mass of 1.344g.

Calculate the value of the integer x and suggest how a student doing this experiment could check that all the water had been removed

Answer 3

reheat, reweigh and check that it is at the constant mass
OR reheat to see if mass is unchanged
OR heat to a constant mass

Question 4

A student heated a solid sample of Na2CO3•xH2O for one minute to remove water and determine a value for x. The experimental value the student obtained for x was lower than the correct value for x. Suggest (1) a reason for this difference and (2) how the procedure could be improved, using the same apparatus, to give a more accurate value for x, justifying your answer

Answer 4

(1) the student did not heat the sample to a constant mass
(2) heat to a constant mass as this will ensure that all the water has been driven off

Question 5

concentration/moles/volume equation

Answer 5

moles (mol) = concentration (mol/dm3) × volume (dm3)

Question 6

ideal gas equation

Answer 6

PV = nRT

• P = pressure (Pa)
• V = volume (m3)
• n = number of moles (mol)
• R = has constant (8.31 - always given)
• T = temperature (K)

Question 7

ideal gas equation unit conversions

Answer 7

• dm3/1000 = m3
• cm3/1,000,000 = m3
• kPa × 1000 = Pa
• MPa × 1,000,000 = Pa
• °C + 273 = K

Question 8

how do you figure out the limiting reagent in a reaction, and how is this used to figure out unknown masses?

Answer 8

1. work out the moles of each product (moles = mass/mr)
2. using the molar ratio (can be seen in the balanced equation), see which is the limiting reagent
3. use the limiting reagent’s number of moles to work out unknown masses

e.g.
332g of nitrogen reacts with 48g of hydrogen - how much ammonia is produced and how much nitrogen is leftover?

Question 9

what are concordant results?

Answer 9

results that are within 0.1 of each other - only use these values to calculate a mean titres

Question 10

how do you calculate the percentage yield of a reaction?

Answer 10

% yield = (experimental mass / theoretical mass) × 100

[i.e. actual mass / what the mass should be according to equations]

Question 11

how do you calculate the percentage atom economy of a reaction?

Answer 11

% atom economy = (Mr of desired product / sum of all Mr of all reactants) × 100

Question 12

when would a reaction have 100% atom economy?

Answer 12

when ONLY the desired product is made, e.g. addition polymerisation reactions

Question 13

in a chemical process, suggest 4 reasons why the yield may be below 100%

Answer 13

• incomplete reaction
• by-products formed
• product loss during washing/purification (e.g. remaining on equipment)
• reaction is reversible

Question 14

how do you calculate the volume needed for diluted solutions?

Answer 14

Question 15

how could you reduce the uncertainty in measuring mass?

Answer 15

use a more precise balance (to more d.p.)

Question 16

how could you reduce the uncertainty in using a burette?

Answer 16

make the titre volume larger by increasing the volume/concentration of substance in conical flask OR decreasing concentration of substance in burette

Question 17

how do you calculate % uncertainty?

Answer 17

% uncertainty = +/- (uncertainty / measurement made on apparatus) × 100

(for total experimental uncertainty, add up all % uncertainties of all the equipment used)