13.1 To prepare a standard solution of sodium carbonate

Question 1

Describe the appearance of sodium carbonate.

Answer 1

Sodium carbonate is a white solid.

Question 2

Why must the mass of the sodium carbonate dissolved in the water be known precisely?

Answer 2

The mass must be know precisely so that the concentration of the sodium carbonate solution can be calculated accurately.

Question 3

Why is sodium carbonate suitable as a primary standard? Distinguish between a primary standard and a standard solution.

Answer 3

Sodium carbonate is suitable as a primary standard because it can be obtained in a pure and stable state and it dissolves easily in water.

A primary standard is a substance which can be obtained in a stable, pure and soluble solid form so that it can be weighted out and dissolved in water to give a solution of accurately know concentration.

A standard solution is a solution of precisely known concentration.

Question 4

Outline the correct procedures for transferring the sodium carbonate to the volumetric flask.

Answer 4

The main points are:

• Accurately measure the known mass of sodium carbonate using a clock glass.
• Correctly transfer the sodium carbonate to the beaker containing water.
• Wash down traces of sodium carbonate from the clock glass (note filter paper is not suitable).
• Stir the solution to ensure that all solid has dissolved completely.
• Use the glass rod and funnel to transfer solution to the volumetric flask.
• Wash all traces of solution from the beaker, rod and funnel into the volumetric flask.
• With the volumetric flask on a level surface, remove the funnel, add deionised water drop by drop, and bring the bottom of the meniscus to the graduation mark on the volumetric flask.
• Stopper the flask and invert it about 20 times.

Question 5

Why is it necessary to slowly add the solid sodium carbonate to the water in the beaker?

Answer 5

Adding the sodium carbonate slowly to the water helps prevent lumps being formed in the solution.

Question 6

State two benefits of a volumetric flask for making up the solution.

Answer 6

The two benefits are:

(1) The volume of the solution in the volumetric flask is accurately known.
(2) The design of the volumetric flask allows it to be shaken easily to ensure a homogenous mixture, i.e. to ensure that the solution is uniform throughout.