Week 3

Question 1

What is the pH scale?

Answer 1

• Goes from 0 to 14
• 1-6 acidic (reds)
• 8-14 alkali (blues)

Question 2

What is an indicator?

Answer 2

• A dye that changes colour depending on the pH
• A universal indicator would give the colours of the pH scale.
• pH probes are more accurate as they can give you a digital reading

Question 3

What is an acid?

Answer 3

• pH of 1-6
• Forms aqueous solutions
• Form H+ ions in water

Question 4

What is a base?

Answer 4

• Dissolves in water to form a solution with pH of more than 7
• OH- ions in water

Question 5

Show a (word equation) neutralisation reaction?

Answer 5

acid + base = salt + water

Question 6

Show a (half equation) neutralisation reaction?

Answer 6

(H+) + (OH-) = H2O

Question 7

What is a titrations practical used for?

Answer 7

• Used to find how much acid is needed to neutralise an alkali (or vice versa).

Question 8

Carry out a titrations practical?

Answer 8

1) Place the pipette filler onto the end of the pipette, now draw in some of the sodium hydroxide (take more solution than you need)
2) Take the rubber bulb off and place your thumb onto the end of the pipette, move your thumb so that the bottom of the meniscus is touching the line. (Now you know that you have exactly 25cm^3 of sodium hydroxide). Now transfer the 25cm^3 of sodium hydroxide into a clean conical flask
3) Close the tap on the burette, place a funnel onto the end of the burette and fill the burette with your SOLUTION. (Caution: be at eye level so any splashes don’t go into your eye). Make a note of the burette reading for later on.
4) Add a couple of drops of phenolphthalein (indicator) to the sodium hydroxide flask. (should turn pink in alkali solution)
5) Open the burette tap so that the ACID is slowly pouring into the flask, swirl. Add the ACID to the sodium hydroxide flask until there is a permanent colour change (the phenolphthalein should go colourless).
6) Now note the final reading of the burette and perform a subtraction to find out how much ACID you added.
7) This was a rough titration meaning that the volume of ACID added is not accurate, to find accurate results you need to carry out the titration again but when you get close to the volume added, you must add the ACID in drops to find an accurate volume.

Question 9

What indicators could you use for titrations?

Answer 9

• Phenolphthalein
• Litmus
• Methyl orange