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 –HPO^2-₄

(e) H₂C₂O₄ – HC₂O^–₄  

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. 

Write down equations for the reactions: 

Oxidation of chromium(III) chloride with bromine in alkaline solution (KOH). 

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? 

 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 . 

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.  

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. 

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 

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.

Write down equations for the reactions:

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