A volume of 200 cm³ of a 2-normal sodium chloride solution (ρ = 1.10 g cm^-3) was electrolysed at permanent stirring in an electrolytic cell with copper electrodes. Electrolysis was stopped when 22.4 dm³ (at STP) of a gas were liberated at the cathode.

 Calculate the mass percentage of NaCl in the solution after electrolysis.  

Relative atomic masses: A_r(H) = 1; A_r(O) = 16; A_r(Na) = 23; A_r(Cl) = 35.5; A_r(Cu) = 64. 

A gaseous mixture containing two neighbour hydrocarbons of the same homologous series was 14.4 times as dense as hydrogen. This mixture with a volume of 16.8 dm³ was hydrated and 350 g of the solution were obtained when the products of hydration were absorbed in water. Ten grams of this solution were taken and heated in the presence of silver(I) oxide which was prepared from 70 cm³ of a 1 N silver(I) nitrate solution. Unreacted Ag²O was dissolved in an aqueous ammonia solution and a residual precipitate was filtered off. The filtrate was acidified with nitric acid and addition of an excess of sodium bromide to it resulted in 9.4 g of a precipitate. When the mixture of the hydrocarbons that remained unreacted, was mixed with a 50 % excess of hydrogen and transmitted above a heated Pt-catalyst, its resulting volume decreased to 11.2 dm³. Volumes of gases were measured in STP conditions. 

 What hydrocarbons were in the starting mixture? 

Quantitative analysis for carbon and hydrogen was originally carried out using a technique and apparatus (see figure) originally developed in 1831 by the famous chemist Justus Liebig. A carefully weighed sample of organic compound (C) is placed in a combustion tube (A) and vaporized by heating in a furnace (B). The vapours are swept by a stream of oxygen through a heated copper oxide packing (D) and through another furnace (E), which ensures the quantitative oxidation of carbon and hydrogen to carbon dioxide and water. The water vapour is absorbed in a weighed tube (F) containing magnesium perchlorate and the carbon dioxide in another weighed tube (G) containing asbestos impregnated with sodium hydroxide. 

 A pure liquid sample containing only carbon, hydrogen and oxygen is placed in a 0.57148 g platinum boat, which on reweighing weights 0.61227 g.The sample is ignited and the previously weighed absorption tubes are reweighed.The mass of the water absorption tube has increased from 6.47002 g to 6.50359 g, and the mass of the carbon dioxide tube has increased from 5.46311 g to 5.54466 g. 

(a) Calculate the mass composition of the compound. 

(b) Give the empirical formula of the compound.To estimate the molar mass of the compound,1.0045 g was gasified. The volume, measured at a temperature of 350 K and a pressure of 35.0 kPa, was 0.95 dm³. 

(c) Give the molar mass and the molecular formula of the compound. 

(d) Draw possible structures corresponding to the molecular formula excluding cyclic structures, stereo isomers, peroxides and unsaturated compounds.There are about 15 possibilities.Give 10 of them. 

When the compound is heated with a sodium hydroxide solution, two products are formed. Fractional distillation of the reaction mixture yields one of the substances.The other substance is purified by distillation after acidification and appears to be an acid. 

(e) What structures are possible for compound C? 

0.1005 g of the acid are dissolved in water and titrated with a sodium hydroxide solution with a concentration of 0.1000 mol dm^-3. The indicator changes colour on addition of 16.75 cm³ of hydroxide solution. 

(f) What was the original substance C? 

 A solution with a volume of 1.00 dm³ is saturated with lead iodide, PbI₂.The concentration of iodide ions is 2.7 mol dm^-3. Determine the solubility product of PbI₂. 

(a) 3.6 × 10^-6 ; 

(b) 2.0 × 10^-8 ; 

(c) 9.8 × 10^-9 ; 

(d) 2.5 × 10^-9 ; 

(e) 4.9 × 10^-9 . 

Write down equations for the reactions:

Action of chlorine on lime water (Ca(OH)₂) in a cold reaction mixture.  

You are required to determine the concentrations of hydrochloric acid and potassium iodate in the diluted solution containing both. 

Procedure: A solution containing potassium iodate and hydrochloric acid has already been measured into the volumetric flask provided. Fill the flask to the mark with distilled water using the wash bottle, close it with a stopper and shake it thoroughly. Fill the burette with the standard sodium thiosulphate solution using one of the beakers provided. (The exact concentration of the thiosulphate is given on the label of the bottle.) 

(a) First titration Pipette a 10.00 cm³ aliquot (portion) of the solution from the volumetric flask into a glass stoppered conical flask. Dilute it with 10 cm³ of distilled water, add 1 g (a small spatula end-full) of potassium iodide and acidify with 10 cm³ of 10 % sulphuric acid using a measuring cylinder. Titrate immediately the iodine formed with the standard sodium thiosulphate solution until the solution in the flask is pale yellow. Add with a pipette 1 cm³ of starch indicator solution and continue the titration to completion. Repeat the titration twice more and record your readings on the result sheet. 

(b) Second titration Pipette a 10.00 cm³ aliquot of the solution into another glass stoppered conical flask, dilute with 10 cm³ of distilled water, add 1 g of solid potassium iodide,and leave to stand for 10 minutes. Then titrate the iodine formed using the standard sodium thiosulphate solution, adding 1 cm³ of starch indicator solution when the mixture is pale yellow. Repeat the titration twice more, recording your readings on the result sheet. 

Task: Calculate the concentration of the HCl and the KIO₃ in the solution that you prepared by dilution (in mol dm^-3). 

By means of chemical equations express the following chemical reactions: 

(a) [Cr(H₂O)₆]Cl₃ dissolved in water, is mixed with an excess of sodium hydroxide solution. A clear green solution is formed. The colour of the solution changes to yellow when an aqueous hydrogen peroxide solution is added. 

(b) If an aqueous solution of a violet manganese compound is mixed with a hydrogen peroxide solution, the resulting solution is decolourised and a gas is released from it. 

Complete the following equation: H₃AsO₄ + Zn → AsH₃ + Zn²⁺ 

 The reaction is carried out in an acid solution. Fill in the missing particles and balance the reaction equation. 

You will follow the concentration change of one of the reactants by the method of comparative visual colorimetry. From data obtained experimentally plot graphically the change of the reactant concentration in dependence on time. 

1. Making of the comparative colorimetric scale of bromine solution Measure with a syringe into 10 identical test-tubes the following quantities of bromine water (0.01-molar): into the first one –10.0 cm³ ; 2nd – 9.0 cm³; 3rd – 8.0 cm³, ……. 9th – 2.0 cm³; into the tenth one – 1.0cm³. Then add to all the test-tubes (except the first one) distilled water to reach a total volume of 10.0 cm³ in each. Seal the test-tubes with stoppers and mix the solutions. Put the test-tubes in a stand with a white background. Finally calculate the concentration (in mol dm^-3) of bromine in the solutions in all testtubes. 

2. Reaction of bromine solution with formic acid Carry out the reaction by mixing 100.0 cm³ of bromine solution with 1.0 cm³ of 1.00- molar solution of formic acid. Immediately after mixing transfer 10.0 cm³ of the resulting solution to the test-tube identical with that used for colorimetric scale. By comparing the colour shade of the reaction mixture (in one-minute intervals) with that of the solutions in the scale, investigate changes of bromine concentration in dependence on time. Put the data in a table containing time (t) and concentration of Br₂. 

Task: Plot the bromine concentration in dependence on time a read the half-time of the reaction from the diagram.

1. Write the equation for the reaction of bromine with formic acid assuming that the reactants are in stoichiometric amounts. 

2. In analytical chemistry a volumetric solution of bromine can be prepared by dissolving a mixture of bromate and bromide in acid medium. Explain this mode of preparation by means of a chemical equation in ionic form. 

Explain by means of generally used international symbols and formulas which compounds are named as peroxo compounds. Write summary formulas for six of them.