PAG 2: Acid-Base Titration

Question 1

What are the steps in preparing a standard solution of known concentration? (6)

Answer 1

1. Weigh out the required mass of solute using a mass balance; 2. Dissolve the solute in a small volume of distilled water in a beaker; 3. Transfer the solution to a volumetric flask using a funnel; 4. Rinse the beaker and funnel with distilled water into the volumetric flask; 5. Fill the volumetric flask to the graduation mark with distilled water; 6. Stopper the flask and gently invert several times to ensure uniform concentration

Question 2

What are common errors in preparing a standard solution? (3)

Answer 2

Systematic errors on the mass balance; Loss of substance during transfer; Overfilling/underfilling the volumetric flask

Question 3

How can errors in preparing a standard solution be reduced? (2)

Answer 3

Ensure all washings are added to the flask; Read volumes from the bottom of the meniscus

Question 4

How should the water be aligned with the graduation line of the volumetric flask? (1)

Answer 4

The bottom of the meniscus should sit on the graduation line

Question 5

What are the steps to performing an acid-base titration? (5)

Answer 5

1. Fill a burette with standard solution; 2. Transfer known volume of unknown solution into a conical flask with pipette; 3. Add a few drops of indicator; 4. Record the initial burette volume; 5. Add standard solution until endpoint and record final volume; 6. Repeat for concordant results

Question 6

How can you use an acid-base titration to calculate the unknown concentration? (4)

Answer 6

1. Calculate moles of standard solution added; 2. Write a balanced equation; 3. Use molar ratio to find moles of unknown; 4. Calculate concentration using moles and volume

Question 7

What is an acid-base indicator? (2)

Answer 7

A weak acid with one colour in its acid form (HIn) and another in its conjugate base form (In⁻)

Question 8

What is the equilibrium expression for an indicator? (1)

Answer 8

HIn ⇌ H⁺ + In⁻

Question 9

When is the end point of a titration reached using an indicator? (1)

Answer 9

When [HIn] = [In⁻]

Question 10

What does the end point of an indicator represent? (1)

Answer 10

Equal amounts of weak acid and conjugate base are present

Question 11

What is important when choosing a suitable indicator for titration? (1)

Answer 11

The pH of the indicator’s end point must fall within the vertical section of the titration curve

Question 12

What is ideal for the position of the indicator’s end point? (1)

Answer 12

As close as possible to the equivalence point of the titration

Question 13

What are suitable indicators for a strong acid / strong base titration? (2)

Answer 13

Phenolphthalein; Methyl orange

Question 14

What is a suitable indicator for a strong acid / weak base titration? (1)

Answer 14

Methyl orange

Question 15

What is a suitable indicator for a strong base / weak acid titration? (1)

Answer 15

Phenolphthalein

Question 16

What is a suitable indicator for a weak base / weak acid titration? (1)

Answer 16

There is no suitable indicator

Question 17

What colour is methyl orange in acid and alkali? (2)

Answer 17

Red in acid; Yellow in alkali

Question 18

What colour is phenolphthalein in acid and alkali? (2)

Answer 18

Colourless in acid; Red in alkali

Question 19

What colour is bromothymol blue in acid and alkali? (2)

Answer 19

Yellow in acid; Blue in alkali