(a)(i) Draw and label a diagram to illustrate the... | WAEC SSCE - Chemistry - 2006

Question 1 Report

(a)(i) Draw and label a diagram to illustrate the preparation and collection of dry chlorine gas in the laboratory.

(ii) List two uses of chlorine.

(b)(i) Explain why river water flowing through an industrial town may be unsafe for drinking.

(ii) State the use of each of the following substances in water treatment: I. Sand, II. Chlorine, III. Calcium oxide, IV. Alum

(c)Consider the reaction represented by the following equation:

2Na\(_2\)CI\(_{(s)}\) + H\(_2\)SO\(_{4(aq)}\) \(\to\) Na\(_2\)SO\(_{4(aq)}\) + 2HCI\(_{(g)}\)

Calculate the volume of HCI gas that can be obtained at s.t.p. from 5.85 g of sodium chloride. [H = 1, Na = 23, CI = 35.5, Molar volume a 22.4 dm\(^3\) at s.t.p]

(d) Give one example in each case of a (i) metal that is a liquid at room temperature. (ii) non-metal that is a iiquid at room temperature, (iii) gas at room temperature that is monatomic.

(e) State two differences between metals and nom metals with respect to their: (i) physical properties; (ii) chemical properties.

Answer

(a)(i) Laboratory preparation and collection of dry chlorine gas

Chlorine is prepared by warming manganese(IV) oxide with concentrated hydrochloric acid:

\[\mathrm{MnO_2(s)+4HCl(aq)\rightarrow MnCl_2(aq)+Cl_2(g)+2H_2O(l)}\]

Preparation and collection of dry chlorine gas in the laboratory.

The gas is passed through saturated sodium chloride solution to remove hydrogen chloride, then through concentrated sulphuric acid to remove water vapour. Since chlorine is denser than air, it is collected by downward delivery, that is, upward displacement of air. It is not collected over water because it dissolves and reacts in water.

(a)(ii) Uses of chlorine

It is used to sterilise drinking water and swimming pools.
It is used as a bleaching agent for paper pulp and textiles.

(b)(i) River water flowing through an industrial town may contain poisonous industrial effluents, such as acids, oils and heavy-metal salts, as well as sewage and disease-causing microorganisms. It is therefore unsafe for drinking without treatment.

(b)(ii) Uses of substances in water treatment

Substance	Use
Sand	Filtration: removal of suspended solid particles.
Chlorine	Disinfection: kills germs and bacteria.
Calcium oxide	Neutralises acidic water and raises its pH.
Alum	Coagulates fine colloidal particles so that they settle or can be filtered off.

(c) Volume of hydrogen chloride gas

\[\mathrm{2NaCl(s)+H_2SO_4(aq)\rightarrow Na_2SO_4(aq)+2HCl(g)}\]

Molar mass of \(\mathrm{NaCl}=23+35.5=58.5\ \mathrm{g\ mol^{-1}}\).

\[\text{Moles of NaCl}=\frac{5.85}{58.5}=0.100\ \mathrm{mol}\]

From the equation, \(2\) mol of \(\mathrm{NaCl}\) produce \(2\) mol of \(\mathrm{HCl}\). Therefore, moles of \(\mathrm{HCl}=0.100\ \mathrm{mol}\).

\[\text{Volume of HCl at s.t.p.}=0.100\times22.4=\boxed{2.24\ \mathrm{dm^3}}\]

(d)

Mercury is a metal that is liquid at room temperature.
Bromine is a non-metal that is liquid at room temperature.
Argon is a monatomic gas at room temperature.

(e) Differences between metals and non-metals

Property	Metals	Non-metals
Physical	Good conductors of heat and electricity; malleable and ductile.	Usually poor conductors of heat and electricity; brittle when solid.
Chemical	Lose electrons to form positive ions and generally form basic oxides.	Gain or share electrons and generally form acidic oxides.

Read lesson note on Non-metals And Their Compounds (JAMB)

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