Acids, Alkalis and Titrations

Question 1

What does the indicator litmus turn when in contact with acids and alkalis?

Answer 1

Acids: red
Neutralisation point: purple
Alkali: blue

Question 2

What does the indicator methyl orange turn when in contact with acids and alkalis?

Answer 2

Acids: red
Neutralisation point: orange
Alkalis: yellow

Question 3

What does the indicator phenolphthalein turn when in contact with acids and alkalis?

Answer 3

Acids: colourless
Neutralisation point: just changing from colourless to pink/pink to colourless
Alkalis: pink

Question 4

What does an acid and metal react together to make?

Answer 4

A salt + hydrogen

Question 5

What does an acid and base react together to make?

Answer 5

A salt + water

Question 6

What does an acid and carbonate react together to make?

Answer 6

A salt + carbon dioxide

Question 7

What classifies as a:
Strong acid 
Weak acid
Neutral
Weak alkali
Strong alkali
on the pH scale?

Answer 7

0-3
4-6
7
8-10
11-14

Question 8

What do alkalis do to acids?

Answer 8

Neutralise them

Question 9

What do acids dissociate in aqueous solutions to produce?

Answer 9

Hydrogen ions (H+)

Question 10

What do alkalis produce in aqueous solutions?

Answer 10

Hydroxide ions (OH-)

Question 11

What do acids reacting with alkalis in a neutralisation reaction produce?

Answer 11

They produce water (the H+ ions from the acid reacting with the OH- ions from the alkali).

Question 12

How does one carry out an acid-alkali titration?

Answer 12

1. A pipette is used to accurately measure a volume of an alkali, often 25 cm3. A pipette filler is used to draw solution into the pipette safely. The alkali is emptied into a conical flask.
2. A few drops of a suitable indicator are then added to the conical flask. This will show a change of colour when the acid and alkali have neutralised one another and the titration is complete.
3. The acid is placed in a burette and the starting volume of acid is read against the scale marked on the burette.
4. The acid from the burette is added to the conical flask, and the flask is swirled to mix its contents. When the acid in the burette has almost run in, it is added one drop at a time. Eventually, a colour change shows that the correct amount has been added to react completely with the alkali in the conical flask.
5. The volume of acid added from the burette is noted. The titration results can then be used to calculate the concentration of the acid or alkali (if the concentration of the other is known).

Universal indicator is unsuitable for titrations because it has a range of colours. Phenolphthalein is often used instead. It changes from pink in alkali to colourless in acid.

Question 13

How many titrations does one need to carry out (minimum)? Why?

Answer 13

Four.
First: Find the approximate end point.
Second: find the exact end point (drop by drop near the end).
Third: ensure reliability.
Forth: without indicator to obtain a pure salt solution.

Question 14

What are the rules for displaying the results of a titration?

Answer 14

Anything with a difference of -1 is considered equal. This is when they’re concordant.
Always read off results from the bottom of the meniscus. If not specified, give results to 2 d.p.