2.0 THE MOLE: Empirical and Molecular formula

2.7 Preparation of molar solutions

Questions 2.7(a)

Accurate measurements are extremely important in Chemistry

1. Name the two quantities we need to measure in order to prepare an aqueous solution of accurate concentration in mol/dm³.
2. Which instruments and apparatus do we require?
3. Which two substances do we require?
4. How can we ensure that the solution we prepare is not contaminated with other substances?

Answers to Questions 2.7(a)

To prepare a solution:

1. Decide on the concentration required, whether 0.1M, 0.5M, 1M, or 2M and so on. Then calculate the mass, M, (if using a solid) that should be dissolved to make 1 dm³ of solution.
2. Measure M accurately on an electrical balance and transfer it into a volumetric flask.
3. Add distilled water to half-fill the bulb of the volumetric flask and shake the mixture to dissolve the solid. Add more water till the level is just below the mark.
4. Carefully top up the level upto the mark using distilled water from a wash bottle. Bottom of the meniscus should be exactly at the mark.

If we measure 80.000 g (2 moles) of sodium hydroxide pellets, dissolve it water and top it up to 1000 ml in volumetric flask, we obtain exactly 2M solution. Because its concentration is known accurately, we call it a standard solution. Suppose we use a 250-ml volumetric flask, what mass of sodium hydroxide would we dissolve in it to make a 2M solution? (Formula = NaOH; Na = 23, O = 16; H = 1)

When using a liquid substance such as concentrated sulphuric (VI) acid, nitric (V) acid and hydrochloric acid, measure volume (instead of mass) using a measuring cylinder. Pour the measured sample into distilled water but not the other way round. Preferably, use a large beaker and let the mixture cool before transferring it into the volumetric flask. Top up the solution up to the mark using distilled water from a wash bottle.

Questions 2.7(b)

1. Sulphuric (VI) acid is normally poured carefully into water when preparing its aqueous solution and not the other way round. Explain this. (Hint: Sulphuric acid produces large quantities of heat (highly exothermic) as it dissolves in water. Water has a lower boiling point than the acid).
2. Explain why the limb of a volumetric flask is made narrow.
3. What is the advantage of using a wash bottle when topping up the level of a solution?
4. Concentrated sulphuric (VI) acid has a density of 1.836 g/ml. What volume of the acid should be used to prepare 250 ml of 0.2M solution?
5. What is the volume of water in 250 ml of 0.2M sulphuric acid?
6. Suggest a reason why, for liquid solutes, we usually measure volume rather than mass when preparing their solutions.

Answers to Questions 2.7b

Measurements of small volumes such as 9.00 ml require use of a small and narrow measuring cylinder (10 ml or 20 ml), because it is sensitive and accurate. Unlike concentrated hydrochloric and nitric acids, concentrated sulphuric (VI) acid is not a solution in water. It is a pure liquid. Its state symbol is therefore (l), not (aq).