2.1.3 - Amount of substance

Question 1

State what is meant by the term molecular formula. (1)

Answer 1

(Actual) number of atoms of each element present in a molecule

Question 2

State what is meant by the term empirical formula. (1)

Answer 2

• The simplest whole number ratio of atoms (of each element) present in a compound

Question 3

Suggest why the student takes four readings of the mass of the crucible and residue to find out the formula of a hydrated salt . (1)

Answer 3

• To make sure all the water had been removed

Question 4

Suggest two modifications to the method that would reduce the percentage uncertainty in the mass of the residue. (2)

Answer 4

• Use balance that weighs to 3/more decimal places
• Use a larger mass (of the named hydrated salt)

Question 5

Suggest one modification that the student could make to their method to reduce the percentage error in the mass of water removed. (1)

Answer 5

• Use balance weighing to 3/more decimal places
• Use a larger mass/amount

Question 6

The student is not sure that all the water of crystallisation has been removed. How could the student modify the experiment to be confident that all the water of crystallisation has been removed? (1)

Answer 6

• Heat to constant mass

Question 7

Describe and explain the change in the rate of the reaction during the first 200 seconds of the experiment.
The figure shows the mass of reactants decreasing over time (2)

Answer 7

• Rate of reaction decreases and concentration decreases / reactants are used up
  ‘allow reaction slows down and concentration of reactants decreases’
• less frequent collisions

Question 8

Outline a method that could be used to obtain the results that are plotted on the graph. Your answer should include the apparatus required and the procedure for the experiment. (3)

Answer 8

• Flask/beaker, balance and stopwatch/ stop clock
• Records mass at time intervals
• Time interval quoted between 10-50s

Question 9

The student plans to repeat the experiment using the same mass of a Group 2 metal from further down the group. Predict whether the volume of hydrogen produced would be greater than, less than or the same as the volume in the first experiment. Explain your answer. (1)

Answer 9

• Less (volume / products)
• Smaller amount / fewer moles / fewer atoms of the metal or element reacting

Question 10

Explain the potential danger of incomplete combustion. (1)

Answer 10

• CO is toxic

Question 11

The student did not obtain the volume of gas predicted in (i) using this procedure. Apart from further repeats, suggest two improvements to the practical procedure that would allow the student to obtain a more accurate result. (2)

Answer 11

• Heat until syringe stops moving / no further gas produced
• Wait until the gas has cooled (to room
  temperature) before measuring the volume