Copper becomes green when exposed to moist air for longer period –

because of the formation of a layer of cupric oxide on the surface of copper
because of the formation of a layer of basic carbonate of copper on the surface of copper
because of the formation of a layer of cupric hydroxide on the surface of copper
because of the formation of a layer of cupric acetate on the surface of copper

Solution

2Cu + H₂O + CO₂ + O₂ → CuCO₃.Cu(OH)₂
(green coloured basic carbonate of copper is formed)

Calomel (Hg₂Cl₂) on reaction with ammonium hydroxide gives

HgO
Hg₂O
NH₂– Hg – Hg – Cl
Hg₂NH₂Cl

Solution

Hg₂Cl₂ + 2NH₄OH → Hg₂NH₂Cl + NH₂Cl+2H₂O

When the same amount of zinc is treated separately with excess of H₂SO₄ and excess of NaOH, the ratio of volumes of H₂ evolved is

1 : 1
1 : 2
2 : 1
9 : 4

Solution

Zn + H₂SO₄ → ZnSO₄ + H₂

Zn + 2NaOH → Na₂ZnO₂ + H₂

1 mole atom of zinc produces the same amount of H₂(2 g) in each case.

The oxidation state of chromium in the final product for med by the reaction between KI and acidified potassium dichromate solution is:

Solution

\(Cr_{2}O_{7}^{2-}+6I^{-}+14H^{-}\rightarrow 3I_{2}+7H_{2}O+2Cr^{3+}\) oxidation state of Cr is +3.

When KMnO₄ acts as an oxidising agent and ultimately forms [MnO₄]^-2, MnO₂, Mn₂O₃, Mn⁺² then the number of electrons transferred in each case respectively is

4, 3, 1, 5
1, 5, 3, 7
1, 3, 4, 5
3, 5, 7, 1

Solution

To prevent corrosion, iron pipes carrying drinking water are covered with zinc. The process involved is

alloy formation
electroplating
galvanising
soldering

Solution

Galvanisation is the process of deposition of zinc metal on the surface of Fe to prevent corrosion.

CrO₃ dissolves in aqueous NaOH to give

\(Cr_{2}O_{7}^{2-}\)
\(Cr_{2}O_{4}^{2-}\)
Cr(OH) ₃
Cr(OH) ₂

Solution

CrO₃ + 2NaOH → Na₂CrO₄ + H₂O

K₂Cr₂O₇ on heating with aqueous NaOH gives

\(CrO_{4}^{2-}\)
Cr(OH)₃
\(Cr_{2}O_{7}^{2-}\)
Cr(OH)₂

Solution

\(Cr_{2}O_{7}^{2-}+2OH^{-}\rightarrow 2Cr_{4}^{2-}+H_{2}O\)

Hence \(CrO_{4}^{2-}\)ion is obtained.

Which of the following elements shows maximum number of different oxidation states in its compounds?

Solution

Among the following, the compound that is both paramagnetic and coloured,is

KMnO₄
CuF₂
K₂Cr₂O₇
All are coloured

Solution

CuF₂ is both paramagnetic and coloured.

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