chemistry Past Questions And Answers

131

The mathematical expression of PH is

A) log 10[OH-]
B) log 101/[H 3 O+]
C) log 10[H 3 O+]
D) log 101/[OH -]

View Discussion (0)JAMB 2014

132

Which of the following gases is alkaline?

A. NO₂
B. CO₂
C. NH₃
D. N₂O

View Discussion (0)WAEC 2022 OBJ

133

(a) (i) Namethree different methods for preparing salts.

(ii) Giveone example of a balanced equation foreach of the methods named in(a)(i).

(iii) Statetwo uses of sodium trioxocarbonate (IV).

(b) If you were given some impure copper, describe how you would

obtain a specimen of the pure metal by electrolysis.

(c) Given that sodium chloride has a solubility of 36.3 at 30 and 39.0 at

100 and that of silver nitrate is 297.0 at 30 and 952.0 at 100.

(i) Calculate the percentage of each substance in the saturated solution at 100 that is deposited on cooling to 30

(ii) Deduce which of thetwo salts can be purified more efficiently by crystallization.

Explanation

(a) (i) - action of an acid upon an active metal / metals higher than hydrogen in the reactivity series

- preparation by double decomposition (reaction between two soluble compounds to produce an insoluble compound)

- by neutralization (reaction between acids & bases)

- action of an acid on trioxocarbonate (IV) of a metal

- direct combination of elements

- action of dilute acid on an insoluble base

Examples:

(ii) - Addition of granulated zinc to a dilute solution of tetraoxosulphate (VI) acid.

Zn(s) + H\(_2\)SO\(_{4(aq)}\) → ZnSO\(_{4(aq)}\) + H\(_{2(g)}\)

- Pb(NO\(_3\))\(_{2(aq)}\) + H\(_2\)SO\(_{4(aq)}\) → PbSO\(_{4(s)}\) + 2HNO\(_{3(aq)}\)

OR

Pb(NO\(_3\))\(_{2(aq)}\) + Na\(_{2}\)SO\(_{4(aq)}\) → PbSO\(_{4(s)}\) + 2NaNO\(_{3(aq)}\)

2NaOH\(_{(aq)}\) + H\(_{2}\)SO\(_{4(aq)}\) → Na\(_2\)SO\(_{4(aq)}\) + 2H\(_2\)O\(_{(l)}\)

ZnCO\(_{3(s)}\) + H\(_2\)SO\(_{4(aq)}\) → ZnSO\(_{4(aq)}\) + H\(_2\)O\(_{(l)}\) + CO\(_{2(g)}\)

- CaCO\(_{3(s)}\) + 2HCl\(_{(aq)}\) → CaCl\(_{2(aq)}\) + H2O\(_{(l)}\) + CO\(_{2(g)}\)

- 2Fe + 3Cl\(_2\) → 2FeCl\(_3\)

Fe + S → FeS

- 2HNO\(_3\) + CuO → Cu(NO\(_3\))\(_2\) + H\(_2\)O

(iii) Uses of Na\(_2\)CO\(_3\)

- used as a water softner

- manufacture of glass/paper/rayon/soaps/detergents/NaOH/Borax

- food processing aid

- pH modifier

- swimming pool chemical

- electrolyte

- analytical reagent / standardization of acids

(b) Use pure copper as the cathode and impure copper as anode and then use a named soluble copper salt e.g. CuSO\(_4\) solution as the electrolyte impure copper dissolves /CuCu\(^{2+}\) + 2e\(^-\) and will be deposited at cathode / Cu\(^{2+}\) + 2e\(^-\) Cu Wash the cathode to obtain pure copper.

(c) (i) Mass of NaCl deposited = 39.0 – 36.3 g = 2.7 g

% NaCl deposited = 2.7 x \(\frac{100}{39.0}\) = 6.92 % or 7.0%

Mass of AgNO\(_3\) deposited = 952.0 – 297.0 = 655 g

% AgNO\(_3\) = \(\frac{655}{952}\) x 100 = 68.8 % Or 69%

(ii) AgNO\(_3\) (Silver trioxonitrate (V)) will be more efficiently crystallized because it has a higher percentage deposited.

View Discussion (0)WAEC 2019 THEORY

134

The number of atoms in one mole of a substance is equal to the

A. mass number
B. oxidation number
C. atomic number
D. avogadro number

View Discussion (0)WAEC 2011 OBJ

135

Which of the following statements about displacement reaction is correct?

A. A more electropositive element displaces a less electroposive one
B. A less electropositive element displaces a more electropositive one
C. The position of elements in the reactivity series has no effect on the reaction
D. it only occurs when the reaction is at equilibrium

View Discussion (0)WAEC 2011 OBJ

136

Given that M is the mass of a substance deposited during electrolysis and Q is the quantity of electricity consumed, then Faraday's first law can be written as [Electrochemical equivalent]

A) M = E/Q
B) M = EQ
C) M = Q/E
D) M = E/2Q

View Discussion (0)JAMB 2011

137

Copper (II) tetraoxosulphate (VI) solution is electrolyzed using carbon electrodes. Which of the following are produced at the anode and cathode respectively ?

A. Copper and oxygen
B. Oxygen and copper
C. Hydrogen and copper
D. Copper and hydrogen

View Discussion (0)JAMB 1992

138

Nuclear fission involves the splitting of a heavy element into two nuclei of

A. nearly equal masses with a relaese of energy and radiaton
B. equal masses with a relaese of energy
C. equal masses with a release of energy and radiation
D. unequal masses without any relaese of energy

View Discussion (0)JAMB 2006

139

Consider the reaction represented by the following Q equation: N\(_2\)O\(_{4(g)}\) \(\rightleftharpoons\) 2NO\(_{2(g)}\) \(\Delta\)H = +57.2KJmol\(^{-1}\)

(a) When is the reaction said to be at equilibrium?

(b) Mention two conditions that can favour the forward reaction

(c) Name the principle involved in (b) above.

View Discussion (0)WAEC 1997 THEORY

140

Group VIl elements in their combined states are called?

A. Halogens
B. Anions
C. Halide
D. Cations

View Discussion (0)WAEC 2020 OBJ