Question

Complete the following sentences by choosing the correct option from the brackets:
(a) The oxide that dissolves in caustic potash (KOH) is [ZnO / CuO]
(b) The reaction that takes place at the anode during the electrolysis of molten lead bromide is [2Br(^-) () Br(_2) + 2e(^-) / 2Br(^-) + 2e(^-) () 2Br(^-)]
(c) The volume occupied by 8 grams of oxygen gas at STP is ______ litres. [15 / 8.96] [Atomic weight of O = 16]
(d) ______ does not give hydrogen gas when it reacts with cold and very dilute nitric acid. [Cu / Mn]
(e) ______ is a polar covalent compound [HCl / CCl(_4)]

Hint:

To remember which oxides are amphoteric, think of ZAP: Zinc, Aluminum, and Pb (Lead) oxides!

Answer 1

(a) The oxide that dissolves in caustic potash (KOH) is ZnO.
Zinc oxide is an amphoteric oxide, so it reacts with both acids and strong bases like KOH.
Copper oxide (CuO) is a basic oxide and does not dissolve in KOH in this way.

(b) The reaction that takes place at the anode during the electrolysis of molten lead bromide is:
2Br^- → Br₂ + 2e^-
At the anode, oxidation takes place, so bromide ions lose electrons and form bromine.

(c) The volume occupied by 8 grams of oxygen gas at STP is 5.6 litres.
Detailed working:
Molecular mass of oxygen gas, O₂ = 32 g
32 g of O₂ occupies 22.4 L at STP
Therefore, 8 g of O₂ occupies:
\[ \frac{8}{32}\times 22.4 = \frac{1}{4}\times 22.4 = 5.6 \text{ litres} \] So the correct value is 5.6 litres.
The bracketed options shown do not match the correct calculation.

(d) Cu does not give hydrogen gas when it reacts with cold and very dilute nitric acid.
Nitric acid is an oxidizing acid, and with copper it does not liberate hydrogen gas.
Instead, oxides of nitrogen are generally formed.

(e) HCl is a polar covalent compound.
In HCl, chlorine is much more electronegative than hydrogen, so the shared pair of electrons is pulled more towards chlorine.
Therefore, HCl has a permanent dipole and is polar.
CCl₄, though it has polar C-Cl bonds, is overall non-polar because of its symmetrical tetrahedral shape.

Final Answers:
(a) ZnO
(b) 2Br^- → Br₂ + 2e^-
(c) 5.6 litres
(d) Cu
(e) HCl

Quick Revision:
ZnO is amphoteric.
Oxidation takes place at the anode.
1 mole of any gas occupies 22.4 litres at STP.
Nitric acid usually does not liberate hydrogen gas with metals like copper.
HCl is polar, while CCl₄ is non-polar overall due to symmetry.