Acids, Alkalis and Titrations

Question 1

What colour is litmus indicator in acid?

Answer 1

Red

Question 2

What colour is litmus indicator in alkali?

Answer 2

Blue

Important: Litmus is not suitable for titrations because it changes colour gradually, but two colour indicators like methyl orange or phenolphthalein are

Question 3

What colour is phenolphthalein indicator in acid?

Answer 3

Colourless

Question 4

What colour is phenolphthalein indicator in alkali?

Answer 4

Pink

Question 5

What colour is methyl orange in acid?

Answer 5

Red

Question 6

What colour is methyl orange in alkali?

Answer 6

Yellow

Question 7

What is the formula for ethanoic acid?

Answer 7

CH₃COOH

Question 8

What is the formula for nitric acid?

Answer 8

HNO₃

Question 9

What three types of substances are considered to be bases/alkalis

Answer 9

Hydroxides, oxides and carbonates

Question 10

What pH is necessary for a substance to be considered a strong acid?

Answer 10

0-3

Question 11

What pH is necessary for a substance to be considered a weak acid?

Answer 11

4-6

Question 12

What pH is necessary for a substance to be considered a neutral?

Answer 12

7

Question 13

What pH is necessary for a substance to be considered a weak alkali?

Answer 13

8-10

Question 14

What pH is necessary for a substance to be considered a strong alkali?

Answer 14

11-14

Question 15

What colour is universal indicator with a strong acid?

Answer 15

Red

Question 16

What colour is universal indicator with a weak acid?

Answer 16

Orange/Yellow

Question 17

What colour is universal indicator with with a neutral solution?

Answer 17

Green

Question 18

What colour is universal indicator with a weak alkali?

Answer 18

Blue

Question 19

What colour is universal indicator with a strong alkali?

Answer 19

Purple

Question 20

What ions are acid a source of when dissolved in an aqueous solution?

Answer 20

H+

(Hydrogen)

Question 21

What ions are alkalis a source of when dissolved in an aqueous solution?

Answer 21

OH-

(Hydroxide)

Question 22

What happens in a neutralisation?

Answer 22

• An acid reacts with an alkali to form a salt and water
• The water is formed due to the hydrogen ion from the acid and the hydroxide ion from the alkali reacting
• For example: HCl + NaOH –> NaCl + H₂O

(Except for ammonia)

Question 23

What are acid-alkali titrations used for?

Answer 23

• Finding the concentration of a solution
• Determining exactly how much acid is needed to neutralise a quantity of alkali and vice versa

Question 24

What are the steps of a titration?

Using the example of hydrochloric acid and sodium hydroxide

Answer 24

• Use a pipette to place exactly 25cm³ of sodium hydroxide which you know the concentration of into a conical flask, and add a suitable indicator to it
• Rinse and fill a burette with hydrochloric acid and measure how much is in the burette, making there there is liquid in the space between the tap and tip
• Add the hydrochloric acid to the conical flask from the burette drop by drop until it changes colour, indicating it has been neutralised
• Swirl the conical flask continuously while doing so
• Measure the new value of how much is in the burette, and calculate the difference to find exactly how much acid has been added
• Repeat twice more or until concordant results have been reached
• For the repeated times, you only need to add it dropwise towards the end

You can do a rough titration first if you want to find out roughly when you need to start going drop by drop

Question 25

How can you use titration results to find out the concentration of the solution being added?

Answer 25

• Find the moles of the solution in the conical flask by doing volume x concentration
• Find the moles of the solution being added by using the ratios inside of the balanced equation
• Do moles/volume to find the concentration