Unit 3-Volumetric Analysis

Question 1

What is a standard solution?

Answer 1

A solution of accurately known concentration.

Question 2

Describe how a standard solution would be made from a primary standard.

Answer 2

1. Accurately measure out required mass of primary standard
2. Dissolve in a small volume of solvent.
3. Transfer solution and rinsing into volumetric flask
4. Make up to graduation mark w

Question 3

What are 4 characteristics of a primary standard?

Answer 3

be available in high state of purity

be stable when solid and in solution

be soluble

have a reasonably high GFM

Question 4

Which of the following chemicals would not be suitable as a primary standard?

Sodium carbonate (Na₂CO₃)

Sodium hydroxide (NaOH)

Hydrated oxalic acid (H₂C₂O₄.2H₂O)

Explain your answer

Answer 4

Sodium hydroxide has a relatively low GFM, is unstable as a solid (absorbs moisture), unstable as a solution.

Question 5

What must be carried out before using sodium hydroxide in a volumetric analysis?

Answer 5

Standardisation

Question 6

How would a solution of sodium hydroxide be standardised?

Answer 6

Titrate with standard solution/solution of known
concentration of a suitable chemical.

For example use oxalic acid for standardising sodium hydroxide or potassium permanganate.

Question 7

What are the four different types of volumetric analysis?

Answer 7

Acid/base titrations

Redox titrations

Complexometric titrations

Back titrations

Question 8

Give an example of the following types of titrations

• Acid/base titrations
• Redox titrations
• Complexometric titrations
• Back titrations

Answer 8

Acid/base titrations-Sodium hydroxide and hydrochloric acid or any non weak/weak pairing.
Redox titrations-Oxalic acid and potassium permanganate or any example based on reactions between oxidising and reducing agents.
Complexometric titrations-based on reactions in which complexes are formed — EDTA and metal ions
Back titrations-useful for solid substances with low solubility calcium carbonate.

Question 9

Why does back titration sometimes need to be used?

Answer 9

For insoluble substances

Question 10

What are the steps in a titration calculation?

Answer 10

Calculate the average titre - 0.1cm³ concordancy

Calculate n using CV [V in litres]

Use mole ratio

Look to see if scaling up required

Calculate the unknown - either m, C, V or %

Question 11

What does EDTA determine in a complexometric titration?

Answer 11

The mass or % of a METAL ion