Related InterviewSolutions

• 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? 
• Write down equations for the reactions: Oxidation of chromium(III) chloride with bromine in alkaline solution (KOH). 
• A mixture of two solid elements with a mass of 1.52 g was treated with an excess of hydrochloric acid. A volume of 0.896 dm3 of a gas was liberated in this process and 0.56 g of a residue remained which was undissolved in the excess of the acid. In another experiment, 1.52 g of the same mixture were allowed to react with an excess of a 10 % sodium hydroxide solution. In this case 0.896 dm3 of a gas were also evolved but 0.96 g of an undissolved residue remained. In the third experiment, 1.52 g of the initial mixture were heated to a high temperature without access of the air. In this way a compound was formed which was totally soluble in hydrochloric acid and 0.448 dm3 of an unknown gas were released. All the gas obtained was introduced into a one litre closed vessel filled with oxygen. After the reaction of the unknown gas with oxygen the pressure in the vessel decreased by approximately ten times (T = const). Write chemical equations for the above reactions and prove their correctness by calculations.  In solving the problem consider that the volumes of gases were measured at STP and round up the relative atomic masses to whole numbers.
• A compound A contains 38.67 % of potassium, 13.85 % of nitrogen, and 47.48 % of oxygen. On heating, it is converted to a compound B containing 45.85 % of potassium, 16.47 % of nitrogen, and 37.66 % of oxygen. Problem: What are the stoichiometric formulas of the compounds? Write the corresponding chemical equation. 
• 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). 
• An amount of 20 g of cooper (II) oxide was treated with a stoichiometric amount of a warm 20% sulphuric acid solution to produce a solution of copper (II) sulphate. How many grams of crystalline copper(II) sulphate (CuSO4 . 5 H2O) have crystallised when the solution is cooled to 20 °C? Relative atomic masses: Ar(Cu) = 63.5; Ar(S) = 32; Ar(O) = 16; Ar(H) = 1 Solubility of CuSO4 at 20 oC: s = 20.9 g of CuSO4 in 100 g of H2O.  
• 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: Ar(H) = 1; Ar(O) = 16; Ar(Na) = 23; Ar(Cl) = 35.5; Ar(Cu) = 64. 
• The IUPAC name of the compound is: (a) 5-bromo-1-hexanoic acid (b) 5-bromo-2-hydroxy-1-hexanal (c) 2-bromo-5-hydroxy-6-hexanal (d) 2-bromo-2-hydroxy-1-hexanal (e) 5-bromo-2-hydroxy-1-hexanone 
• Which of the following acid-base pairs is most suitable for keeping the pH constant at 9 in an aqueous solution? (a) CH3COOH – CH3COO–   (b) NH+4 – NH3(c) H2CO3 –HCO-3(d) H2PO-4 –HPO2-4(e) H2C2O4 – HC2O-4  
• 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.