Chemistry, Third edition

152 9 · CALCULATING CONCENTRATIONS

9.1.What mass of solute must be used in order to prepare the

following solutions?

(i) 250 cm^3 of 0.200 mol dm^3 sodium hydrogencarbonate

(NaHCO 3 (aq))?

(ii)2.50 dm^3 of 0.500 mol dm^3 potassium nitrate (KNO 3 (aq))?

(iii)750 cm^3 of 0.050 mol dm^3 oxalic acid

(H 2 C 2 O 4 ·2H 2 O(aq))?

9.2.Standard solutions of sodium hydroxide are usually

obtained in the laboratory by dilution of a commercial stan-

dard solution, or they are standardized by acid–base titrations.

Why do you think that solid sodium hydroxide is not very

suitable to use as a primary standard? (Hintwhat happens to

solid sodium hydroxide in air?)

9.3.What volumes of the following solutions will neutralize

50 cm^3 of 0.10 mol dm^3 sulfuric acid?

(i)0.20 mol dm^3 potassium hydroxide solution

(ii)0.05 mol dm^3 of sodium carbonate solution.

9.4.Lead(II) nitrate solution (Pb(NO 3 ) 2 ) reacts with potassium

iodide solution (KI) according to the following ionic equation:

Pb^2 (aq) + 2I(aq)PbI 2 (s)

(i)What volume of 1.00 mol dm^3 KI(aq) is needed to react

exactly with 25.0 cm^3 of Pb(NO 3 ) 2 (aq) of concentration

2.00 mol dm^3?

(ii)This reaction can be used for the gravimetric determination

of lead(II) ions in solution. What mass of PbI 2 (s) would be

precipitated from the lead(II) nitrate solution in (i)? (Use

accurate atomic masses in this calculation.)

9.5.What mass of the following will react with 25 cm^3 of

0.250 mol dm^3 nitric acid?

(i)solid potassium hydroxide (KOH)

(ii)solid copper carbonate (CuCO 3 ).

9.6.Sodium thiosulfate (Na 2 S 2 O 3 ·5H 2 O) solution reacts with

aqueous iodine in a redox reaction according to the equation

2S 2 O 32 (aq) + I 2 (aq)S 4 O 62 (aq) + 2I(aq)

A solution, of volume 500 cm^3 , contains 12.400 g of dissolved

hydrated sodium thiosulfate.

(i)What is the concentration of the solution in mol dm^3?

(ii)11.8 cm^3 of this solution reacted exactly with 25 cm^3 of a

solution of aqueous iodine. What is the concentration of

the iodine solution in g dm^3?

9.7.How much of a standard solution of Ca^2 (aq) of concen-

tration 1000 ppm should you take and dilute in order to form

2dm^3 of a solution of concentration 50 ppm?

9.8.A bottle of aqueous hydrochloric acid (HCl) has 35%

(w/w) on the label. What mass of solution contains 7.0 g of HCl?

9.9.Complete this table:

Substance Molar concentration Metal ion concentration

/mol dm^3 /ppm or mg dm^3

KBr 0.050

Na 2 SO 4 0.010

Pb(NO 3 ) 2 0.0060

9.10.Calculate the mass (in grams) of lead in 200 cm^3 of a

solution of lead nitrate that contains 100 mg of lead nitrate per

dm^3 of solution.

9.11.What mass of Na 2 CO 3 · 10H 2 O is required to make

1dm^3 of a 20 ppm solution of Na(aq)?

9.12.(i)Calculate the pH of a solution whose [H(aq)]

4.48 10 ^9 mol dm^3.

(ii)Calculate the hydrogen ion concentration (in mol dm^3 )

for a solution with a pH value of 5.93.

Revision questions

Extension material to support this unit is available on our website. For extension material concerning

gas concentrations and pH see Appendix 9 on the website.