Evaluative coursework 1

Question 1

What would be the observed colour change if dichromate ions were used to oxidise a species?

Answer 1

Orange to green.

Question 2

What is a test for a hydrated salt?

Answer 2

• Upon heating, the salt should decrease in mass as the water of crystallisation evaporates.- The gas released should turn blue cobalt chloride pink, indicating the presence of water.

Question 3

What would be the observed colour change if manganate ions were used ti oxidise a species?

Answer 3

Pink/purple to colourless.

Question 4

How would the percentage error for a titration be calculated?

Answer 4

• The absolute uncertainty for one reading from the burette is +/- 0.05cm3.- Each titre is obtained by subtracting initial reading from final reading, so 2 readings are made, thus absolute uncertainty for each titre is 2x0.05cm3 = 0.1cm3.- Percentage error = (Absolute error (0.1) / Mean titre) x 100.

Question 5

What modifications could be made to reduce percentage uncertainty in titration?

Answer 5

1. Use more concentrated solution being titrated so that average titre is greater.2. Use larger volume of same concentration of solution being titrated so that average titre is greater.3. Use more dilute titrating solution so that greater volume is needed for same volume of solution being titrated, resulting is greater average titre.

Question 6

How do you ensure all water of crystallisation has evaporated when heating a hydrated salt?

Answer 6

Heat and weigh the salt, repeat process until there is no further change in mass of salt.

Question 7

How could you reduce the percentage error when weighing a mass of heated salt?

Answer 7

• Use a more accurate scale (3 d.p. scale).- Use a greater initial mass of hydrated salt.

Question 8

What would be the consequence of overshooting a titration for the final value of x?

Answer 8

• x would be higher.- Overshooting titration means using greater volume of solution to react with solution being titrated.- This would decrease concentration of solution since concentration = mol/volume. - There would be a decrease in no. of moles in original solution since mol = volume x conc.- M (molar mass) of original hydrated salt would increase since M = mass/mol.

Question 9

What would be the consequence of overheating a hydrated salt to the points of its thermal decomposition on the final value of x?

Answer 9

• x would be higher.- Overheating salt to the point of decomposition would decrease final mass of the salt. Since mass of water lost = Initial mass - final mass, mass of water lost would decrease.- Since mol of water = mass of water lost/M of water, mol of water would increase.

Question 10

What are some procedural errors for the titration?

Answer 10

1. The salt used may not have been fully hydrated. Loss of water may have caused final value of x to be smaller than actual value.2. The end point of titration may not have been completely colourless so it may have been easy to overshoot titration.

Question 11

What are some procedural errors for heating the salt?

Answer 11

1. Not all of the water could have been driven off, giving smaller value of x than expected.