A free sample: two questions from each of the five most recent solved papers, with marking-scheme solutions.
A non-metal which reacts with concentrated sulphuric acid to form two gases which turn lime water milky is ............... .
Carbon
Reason — Carbon (a non-metal) reacts with hot concentrated sulphuric acid to form two gases — carbon dioxide (CO2) and sulphur dioxide (SO2). Both these gases turn lime water milky.
C + 2H2SO4 [conc.] ⟶ CO2 + 2H2O + 2SO2
Alex was given a solution of an unknown salt Y for analysis. He performed the following tests and recorded his observations:
Based on the observations made by Alex, identify:
(a) the anion and
(b) the cation present in salt Y.
(a) Anion — Chloride (Cl-)
(b) Cation — Copper(II) (Cu2+)
Reason —
(a) The white precipitate formed with silver nitrate solution which is soluble in ammonium hydroxide is silver chloride (AgCl). This indicates the presence of chloride ion (Cl-) as the anion.
AgNO3 + Cl- ⟶ AgCl ↓ [white ppt.] + NO3-
AgCl + 2NH4OH ⟶ [Ag(NH3)2]Cl + 2H2O
(b) The pale blue precipitate formed with sodium hydroxide solution is copper(II) hydroxide [Cu(OH)2]. This indicates the presence of copper(II) ion (Cu2+) as the cation.
Cu2+ + 2NaOH ⟶ Cu(OH)2 ↓ [pale blue ppt.] + 2Na+
Hence, the salt Y is copper(II) chloride (CuCl2).
Which gas decolourises potassium permanganate (KMnO4) solution?
Sulphur dioxide
Reason — Sulphur dioxide gas turns acidified potassium permanganate from pink to clear colourless.
2KMnO4 + 2H2O + 5SO4 ⟶ K2SO4 + MnSO4 + H2SO4
The atomic number of two atoms ‘X’ and ‘Y’ are 14 and 8 respectively.
State:
(a) the period to which ‘X’ belongs.
(b) the formula of the compound formed between ‘X’ and ‘Y’.
(Do not identify X and Y)
(a) The atomic number of X is 14
Electronic configuration ⟶ 2, 8, 4
Number of electron shells = 3, hence X belongs to period 3
(b) The atomic number of Y is 8
Electronic configuration ⟶ 2, 6
X has a valency of 4 and Y has valency of 2
$$\begin{aligned}\overset{\phantom{4}{4}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{Y}} \Rightarrow \underset{\phantom{2}{2}}{\text{X}} \space {\swarrow}\mathllap{\searrow} \underset{\phantom{4}{4}}{\text{Y}} \\[1em] \text{X}_2\text{Y}_4\end{aligned}$$
∴ Simplified formula of the compound is XY2
Unsaturated hydrocarbons undergo:
Addition reaction
Reason — The availability of electrons in the double or triple bonded unsaturated hydrocarbons makes them more reactive and they therefore undergo characteristic addition reactions only.
Rewrite the following statements by adding the correct word as shown in the example:
Example:
Given Statement: Ammonia changes moist red litmus to blue.
Correct Statement: Aqueous ammonia changes moist red litmus to blue.
(a) Sulphuric acid acts as a dehydrating agent.
(b) Ammonia reacts with chlorine to give ammonium chloride and nitrogen.
(a) Concentrated sulphuric acid acts as a dehydrating agent.
(b) Excess of ammonia reacts with chlorine to give ammonium chloride and nitrogen.
An element in period 3, whose electron affinity is zero:
Argon
Reason — Argon belongs to period 3. It is an inert element and its octet is complete hence, it's electron affinity is zero.
Identify the cation in each of the following cases:
(a) Ammonium hydroxide solution when added to Solution B gives a white precipitate which does not dissolve in excess of ammonium hydroxide solution.
(b) Sodium hydroxide solution when added to Solution C gives a white precipitate which is insoluble in excess of sodium hydroxide solution.
(a) Pb2+
Reason — When ammonium hydroxide solution is added to lead nitrate, it forms a chalky white precipitate of lead hydroxide [Pb(OH)2] which is insoluble in excess of ammonium hydroxide.
Pb(NO3)2 + 2NH4OH ⟶ 2NH4NO3 + Pb(OH)2
(b) Ca2+
Reason — When sodium hydroxide solution is added to calcium nitrate, a white precipitate of calcium hydroxide is obtained which is insoluble in excess of sodium hydroxide solution.
Ca(NO3)2 + 2NaOH ⟶ Ca(OH)2 ↓ + 2NaNO3
The element with highest ionization potential, is:
Helium
Reason — As ionization potential decreases down the group and increases on moving from left to right in a periodic table, hence, helium, the element on the top right most corner of the periodic table has highest ionization potential.
The inert electrode used in the electrolysis of acidified water, is:
Platinum
Reason — The inert electrode used in the electrolysis of acidified water is Platinum.
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