General information

Question 1

What is the main hazard associated with dilute acids and alkalis and what safety precaution should be taken

Answer 1

They are irritants- wear goggles

Question 2

What is the main hazard associated with strong acids and alkalis and what safety precaution should be taken

Answer 2

They are corrosive- wear goggles

Question 3

What safety precaution should be taken when using flammable chemicals

Answer 3

Keep away from naked flames

Question 4

What safety precaution should be taken when using toxic chemicals

Answer 4

• wear gloves
• avoid skin contact
• wash hands after use

Question 5

What safety precuation should be taken when using oxidising chemicals

Answer 5

Keep away from flammable or easily oxidised materials

Question 6

What method could be used for measuring mass loss in thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen

Answer 6

Heating in a crucible

Question 7

Describe the method for determining the water of crystallisation of a compound by heating in a crucible

Answer 7

• Weigh an empty clean dry crucible and lid.
• Add 2g of hydrated calcium sulfate to the crucible and wiegh again.
• Heat strongly with a Bunsen for a couple of minutes.
• Allow to cool.
• Weight the crucible and contents again.
• Heat crucible again and reweigh until you reach a constant mass.

Question 8

Why must a lid be used when heating a compound in a crucible to measure mass change

Answer 8

• It improves the accuracy of the experiment as it prevents the loss of solid from the crucible.
• This lid should be loose fitting to allow gas to escape.

Question 9

Why shouldn’t very small or very large masses of solid be used when heating in a crucible to measure mass change

Answer 9

• Large amounts of solids should not be used as the decomposition is likely to be incomplete.
• Small amounts of solids should not be used as the percentage uncertainties in weighing will be too high.

Question 10

Why must the crucible be dry when heating a compound to measure its mass change

Answer 10

A wet crucible would give an inaccurate result- it would cause the mass loss to be too large as the water would be lost when heating.

Question 11

How do you work out the value of X in a water of crystallisation question

Answer 11

• Subtract final mass of compound (after heating) from initial mass of the hydrated compound.
• This is the mass of water- divide it by the mr of water (18) to work out the moles of water.
• Divide the final mass of the compound (after heating) by the compounds mr to work out its moles.
• Divide the moles of water by the moles of the compound to work out the value of X.

Question 12

What are the potential sources of error when using a gas syringe

Answer 12

• gas escapes before bung inserted
• syringe sticks
• some gases like carbon dioxide or sulphur dioxide are soluble in water so the true amount of gas is not measured

Question 13

Decribe the method for using a gas syringe to calculate the mr of propanone

Answer 13

1. Extract 0.2 cm^3 of propanone into a hypodermic syringe and then measure the mass of this syringe.
2. Using hand protection, remove a gas syringe from the oven and note the volume of air already in the barrell- about 5cm^3
3. Inject the propanone through the self-seal cap into the barrel. The plunger will move straight away.
4. Put the gas syringe back into the oven
5. Measure the mass of the empty hypodermic syringe immediately.
6. After a few minutes measure the volume of the gas in the gas syringe, record the temperature of the oven shelf and the pressure of the room.
7. Use the ideal gas equation.

Question 13

What are two key things to remember when drawing a diagram of a gas syringe

Answer 13

• make sure you don’t leave gaps in your diagram where gas could escape
• make sure you draw the syringe with some measurement markings on the barrel