Amount of substance

Question 1

The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette.
Suggest why this might affect the titre volume recorded.
[1 mark]

Answer 1

Additional drops of solution could have entered the burette from the funnel, (making the value on the burette lower).

Question 2

State one advantage of using a conical flask rather than a beaker for the titration.
[1 mark]

Answer 2

Less chance of splashing/losing any solution using a conical flask
(when swirling)

Question 3

formula for no of particles

Answer 3

No of particles =
moles of substance (in mol) x Avogadro’s constant

Question 4

Define empirical formula

Answer 4

An empirical formula is the simplest ratio of atoms of each element in the compound.

Question 5

Define molecular formula

Answer 5

A molecular formula is the actual number of atoms of each element in the compound.

Question 6

formula for % yield

Answer 6

percentage yield =
(actual / theoretical) x 100

Question 7

formula for % atom economy

Answer 7

(mass of useful product/ mass of all reactants) x 100

Question 8

formula for % uncertainty

Answer 8

(uncertainty / measurement made on apparatus) x 100

Question 9

In a titration which used a pipette to measure 25 cm3 with an error of +/- 0.05 and a burette
that measured 30cm3 with an error of +/- 0.1
Calculate the total error

Answer 9

PE for piette = 0.05/25 x100 = 0.2%
PE for burette = 0.1/30 x100 = 0.33%
Total error in the titration = 0.2 + 0.33 = 0.53%

Question 10

How do you calculate empirical formula

Answer 10

Step 1 – Divide the mass of each element by the Ar of that element to work out the number
of moles.
Step 2 – Which ever element has the smallest number of moles, divide all others by that
value.
Step 3 – This will enable you to determine the simplest ratio of elements.

Question 11

Chlorine, Cl2, reacts with Sodium Hydroxide, NaOH to form Sodium Chloride, NaCl, Water,
and Sodium Chlorate, NaOCl. Sodium Chlorate is the desired product from this reaction as it
is used in bleach. Calculate the % atom economy for this reaction.
2NaOH + Cl2  NaCl + NaOCl + H2O

Answer 11

‘Mr of desired product(s)’ is the Mr of NaOCl
= 23.0 + 16.0 + 35.5 = 74.5
‘Total Mr of all reactants’ is 2 x Mr of NaOH added to Mr of Cl2
= 2 x (23.0 + 16.0 + 1.0) + 35.5 + 35.5 = 151.0
% Atom Economy = 74.5/151 x 100 = 49.3%

Question 12

13.5g of Aluminium reacts with an excess of Oxygen from the atmosphere to form Aluminium Oxide (Al2O3). 18.4g of Aluminium Oxide was produced. Calculate the percentage
yield.
2Al + 1½ O2 -> Al2O3

Answer 12

Moles of Al = Mass = 13.5/27(Mr) = 0.5 moles
Al2O3 mole ratio 2:1 so 0.5/2 = 0.25
Mr of Al2O3 = 102 x 0.25 = 25.5
18.4 (actual mass) / 25.5 (theo) x 100 = 72.2%

Question 13

Steps to make up a standard solution:

Answer 13

Stage 1: transfers known mass of solid
a) Weigh the sample bottle containing the solid on a (2 dp) balance
b) Transfer to beaker* and reweigh sample bottle
c) Record the difference in mass

Stage 2: Dissolves in water
a) Add distilled / deionised water
b) Stir (with a glass rod) or swirl
c) Until all solid has dissolved

Stage 3: Transfer, washing and agitation
a) Transfer to volumetric / graduated flask using a funnel
b) With washings
c) Make up to 250cm3 / mark with water
d) Shakes/inverts/mixes

Question 14

Importance of percentage yield

Answer 14

• idea of maximising the mass of reactants / atoms that ends up in desired product or idea of minimising the amount of by-products