(a)(i) Define the term hygroscopic.
(ii) Give two difference: between a physical change and a chemical change.
(iii) Using the kinetic theory of gases, explain briefly the Charles’ law.
(b)(i) Arrange the following compounds in order of increasing boiling points: CS\(_2\); CO\(_2\); NaH. Give reasons for your answer.
(ii) Write a balanced chemical equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
(c) In a certain reaction, 15.0 g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6 dm\(^2\) of hydrogen gas at s.t.p.
(i) Write a balanced equation for the reaction.
(ii) Calculate the: I. mass of pure magnesium in the sample; I. percentage purity of the magnesium sample; III. number of ions produced in the reaction. [Mg = 24.0; volume at s.t.p. 22.4 dm\(^{-3}\), Avagadro’s constant = 6.02 x 10\(^{23}\)mol\(^{-1}\)]
Explanation
(a)(i) Define the term Hygroscopic: Hygroscopic substance – is defined as a substance which absorbs moisture on exposure to the atmosphere but does not form solution or change state.
(ii) Give two differences between a physical and a chemical change:
| Physical Change | Chemical Change |
| No new substance formed |
New substance formed |
|
Change is easily reversibl |
Change is not easily reversible |
|
Heat change is not usually involved |
Heat is usually given out or absorbed/it involves heat |
|
No change in mass of substance involved |
Change in mass of substance involved |
(iii) Using the kinetic theory of gases, explain briefly the Charles' law: At constant pressure, when the temperature of a gas is increased, the molecules gain more kinetic energy and move faster. The distance between the molecule increases resulting in increased in the volume of the gas.
(b)(i) Arrange the following compounds in order of increasing boiling points:
CO\(_2\), CS\(_2\), NaH
CO\(_2\) — Weak vander Waal's forces.
CS\(_2\) — Stronger vander Waals's forces.
NaH — With ionic forces stronger than vander Waal's forces.
(ii) Write a balanced equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
Cl\(_{2(g)}\) + 2NaOH\(_{(aq)}\) \(\to\) NaCl\(_{(aq)}\) ; NaCI\(_{(g)}\) + H\(_2\)O\(_{(l)}\)
(c) In a certain reaction, 15.0g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6dm\(^3\) of hydrogen gas at s.t.p
(I) Write a balance equation for the reaction.
Mg\(_{(g)}\) 2HCI\(_{(aq)}\) ---> MgCl\(_{2(ag)}\) + H\(_{2(g)}\).
(ii) Calculate the: I. Mass of pure magnesium in the sample
Mg + 2HCI \(\to\) MgCl\(_2\) + H\(_2\)
1 mole H\(_2\) = 1 moles mg
n(H\(_2\)) = \(\frac{8.6}{ 22.4}\)
= 0.384moles.
Hence, number of moles of Mg = 0.384moles
Mass of Mg = mole x molar mass
= 0.384 x 24
= 9.21g.
II. % purity = \(\frac{\text{Mass of pure Mg}}{\text{Mass of impure Mg}}\) x 100
Mg = \(\frac{9.21}{15.0}\) x 100
= 61.4%
Ill. From the equation;
Moles of cl\(^{-2}\) = 2 x moles of Mg
= 2 x 0.384
= 0.768 moles
No of Cl\(^{-}\) produced x N\(_A\)
= 0.768 x 6.02 x 10\(^{23}\)
= 4.62 x 10\(^{23}\) Cl\(^{-}\)