PRECISE VERSUS IMPRECISE CONCENTRATIONS
We do not always need to know the concentrations of solutions used in the laboratory very precisely e.g. bench reagents, test solutions, indicators etc. It is not worth spending time and effort making up solutions if the required precision does not need it
These are solutions used in analytical determinations. These concentrations need to be known precisely (within specified limits). They are made either by weighing out a known mass of a standard solid and making up to a specified volume (e.g. sodium carbonate). A standard solution is one whose concentration is known (to an appropriate number of significant figures, s.f.) and which doesn’t go off with time i.e. is stable.
A standard solution is prepared by dissolving a definite weight of substance (a primary standard), in a definite volume. A substance is classified as a primary standard if it has following characteristics:
(i) It is easily available in state of high purity.
(ii) It is neither hygroscopic nor deliquescent.
(iii) It shows high solubility in water.
(iv) It does not dissociate or decompose during storage.
Substances whose standard solutions cannot be prepared directly are called secondary standards. For example, KMn04, NaOH, KOH, etc.
An alternative approach is to buy small quantities of standard solutions (ready-made or concentrated) and use these to standardise one’s own solutions. Once a solution has been standardised so that we know its concentration precisely, we can use this as a ‘secondary standard’ to standardise other solutions.
So we can use sodium carbonate to standardize hydrochloric or sulphuric acids, which in tum can be used to standardise sodium hydroxide solutions.
By bench reagents we mean dilute acids and bases etc. that are routinely used in the laboratory for tests, simple reactions and preparations. The exact concentration is not important and they can be made up with less care and precision, and do not need standardisation. The concentration is usually reported as 0.5M, 1M, 2M, 4M etc., which implies it is not very precise.
I. Objective Type Questions ——–
-Select the most appropriate choice from the options given as
(a), (b), (c) and (d) after each question:
1. 10 cm3 of which of the following solution contains maximum number of ions assuming each solute to be fully ionised:
(a) 0.2 M NaCI (b) 0.2 M Cr2(SO.J3
(c) 0.2 M CaCl (d) 0.2 M N~S04.
2. An aqueous solution of 6.3 g of oxalic acid dihydrate is made up to 250 cm3. The volume of 0.1 N NaOH required to completely neutralise 10 cm3 of this solution is
(a) 40 cm3 (b) 20 cm3
(c) 10 cm3 (d) 4 cm3.
3. 25 cm3 of a solution of Ba(OH)2 on titration with a 0.1 M solution of HCl gave a titre value of 35 cm3. The molarity of barium hydroxide solution was
(a) 0.07 (b) 0.14
(c) 0.28 (d) 0.35.
4. Molarity of 0.2 N S04 is
(a) 0.2 (b) 0.4
(c) 0.6 (d) 0.1
5. The volume of water to be added to l 00 cm3 of 0.5 N H2SO4 to get decinormal concentration is
(a) 400 cm3 (b) 450 cm3
(c) 500 cm3 (d) 100 cm3.
6. Which among the following is not affected by temperature?
(a) Normality (b) Formality
(c) Molarity (d) Molality.
7. The number of cm3 of 6M HCl which are required to prepare 150 cm3 of solution which is 0.30 Min hydronium ions is
(a) 3.0 (b) 7.5
(c) 9.3 (d) 30.
8. One molar solution of sulphuric acid is equal to
(a) normal solution (b) N/2 solution
(c) 2N solution (d) 4N solution.
9. How many grams of S04 should be present in 100 cm3 of its aqueous solution to produce a solution of 0.1 N?
(a) 0.49 g (b) 4.9 g
(c) 49 g (d) 2.45 g.
10. How much water should be added to 200 cm3 of 0.5 M NaOH to make it exactly 0.4 M NaOH solution?
(a) 250 cm3 (b) 50 cm3
(c) 160 cm3 (d) 100 cm3.
1. How many moles of sodium hydroxide are present in 50 cm3 of its 0.4 M solution?
(a) 0.4 moles (b) 0.04 moles
(c) 0.20 moles (d) 0.020 moles.
12. What is the concentration of a solution containing 15 g potassium hydrogen trioxocarbonate (IV) KHC03, in 3.0 dm3 of its solution?
(a) 0.01 M (b) 0.02 M
(c) 0.04 M (d) 0.05 M.
13. The mass in grams of ammonia in 500 cm3 aqueous ammonia solution of concentration 2.0 mol dm-3 is
(a) 17g (b)l.7g
(c) 3.4 g (d) 34 g.
14. 100 cm3 of 2.5 M NaOH solution is diluted to 250 cm3. The concentration of the diluted solution is
(a) 0.50 M (b) 0.10 M
(c) 1.0 M (d) 2.0 M.
15. Calculate the concentration of stock trioxonitrate (V) acid which has mass density of 1.50 g cm-3 and has purity of 70%
(a) 23.8 M (b) 2.38 M
(c) 17.0 M (d) 16.7 M.
16. How many moles of chloride ions are present in 50 cm3 of 0.50 M calcium chloride, CaCl2, solution?
(a) 0.10 mol (b) 0.02 mol
(c) 0.05 mol (d) 0.20 mol.
II. Fill in the Blanks.
17. Complete the following sentences by supplying appropriate words:
(i) Molality of the solution is number of moles of the solute in each …… kg of ….. .
(ii) The ratio of the number of moles of solvent to the total number of moles of solute and solvent is called ……
(iii) If molarity of oxalic acid solution is M/2 then its normality will be ……
(iv) Aerated drinks in an example of …… in …… solution.
(v) Substances whose standard solution cannot be prepared directly are called …… .
(vi) Concentrated solutions which can be diluted are known as …… solutions.
(vii) Mixing 200 cm3 of 0.50 M NaOH will 100 cm3 of 0.50 M NaOH will give a new solution of …… M.
(viii) The amount of solute in grams present per dm3 of solution is known as …… .
(ix) Amount of water to be added to 200 cm3 of 1 M HCI to make it exactly 0.2 M HCl is …… .
(x) A decimolar solution of NaOH contains …… g NaOH per litre of the solution.
III. Discussion Questions———–
18. Calculate the mol fraction of ethylene glycol (C2H60 2) in a solution containing 20% of C2H60 2 by mass in aqueous solution.
19. Calculate molarity of a solution containing 5.0 g of NaOH in 450 mL of solution. What additional data is required to calculate molarity of solution?
20. Calculate molality and the mol fraction of solute in the following solutions.
(i) A sugar syrup of mass 214.2 g containing 34.2 g of sucrose
(ii) 5 M aqueous solution of A (Molar mass 100 g mol-1) with density 1.289 g cm-3.
21. What volume of decamolar aqueous solution of hydrochloric acid is required to prepare 2.0 dm3 of 5 M HCl solution?
22. 200 cm3 of an aqueous solution containing 8 g of KOH are mixed with 200 cm3 of pentimolar KOH solution. What is the molarity of the resulting solution?
23. What is molarity of the following
(i) Pure water if its density is 1000 kg/m3.
(ii) K+ ions in a solution obtained by mixing 250 cm3 each of 0.2 M KCl and 0.2 M ~S04.
24. Calculate the molarity of each of the following solutions
(a) 30 g of Co(N03) 2.6Hp in 4.3 dm3 of solution. (b) 30 cm3 of 0.5 M H2S04 diluted to 500 cm3.
25. What maximum volume of 1.5 M HCl that can be obtained from 1.0 dm3 each of 0.5 M and 5 M HCl solutions, by mixing them without addition of water?
26. Calculate (a) molality, (b) molarity and (c) mole fraction of KI if the density of 20% (mass/mass) aqueous KI is 1.202 g/cm3.
27. (a) What is a standard solution?
(b) List six pieces of apparatus that would be used in preparing solutions of known concentration.
(c) Outline the main steps to be followed in preparing standard solutions.
(d) Calculate the concentration you would put on a label of a solution prepared by dissolving 8.0 g of potassium hydroxide, KOH, in 500 cm3 of distilled water.
A, (K = 39, 0 = 16.0, H = 1)
28. (a) Write down the relation between mass, m; molar mass, M; and amount of substance, n.
(b) Given that mass concentration, p, is the mass per unit volume, V, in dm3, derive a relation between mass concentration, p, and amount of substance concentration, c.
(c) 9.0 g of the hydrated salt CuS04. Xh20 was dissolved in – 500 cm3 of water and its concentration in mol ctm-3 was experimentally found to be 0.0720 M. Calculate
(i) the molar mass of CuS04.x~O;
(ii) x, the water of crystallization;
(iii) the number of Cu2+ ions in 100 cm3 of the prepared