JEE Chemistry S Block Elements MCQs Set 21

Chemistry Objective Questions and Answers: S Block Elements

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Multiple Choice Questions

Question 1: Which of the following statements about alkali metals are correct? (1) Melting point decreases with atomic number. (2) Potassium is lighter than sodium. (3) Their salts impart characteristic colours to an oxidising flame.
(a) (1) and (2) only
(b) (1) and (3) only
(c) (1), (2) and (3)
(d) (2) and (3) only
Show Answer & Explanation

Answer: (c) (1), (2) and (3)

Explanation:
1. Melting points fall from Li to Cs. K has a lower density than Na (0.86 against 0.97 g/cm3). Li to Cs all give a flame test colour.

Teacher's Note:
1. Density does not rise smoothly: K is lighter than Na.
2. Li crimson, Na yellow, K violet, Rb red violet, Cs blue.

Question 2: On dissolving a moderate amount of sodium in liquid ammonia at low temperature, which of the following does NOT occur?
(a) The liquid ammonia remains diamagnetic
(b) A blue solution is obtained
(c) Na+ ions are formed in solution
(d) Liquid ammonia becomes a good conductor
Show Answer & Explanation

Answer: (a) The liquid ammonia remains diamagnetic

Explanation:
1. Dilute solutions contain ammoniated electrons, which have unpaired spin, so the solution is paramagnetic.

Teacher's Note:
1. The blue colour and the conductivity come from the ammoniated electron.
2. Only very concentrated solutions become diamagnetic.

Question 3: The incorrect statement is
(a) KOH can be used to absorb carbon dioxide
(b) liquid sodium is used as a coolant in fast breeder reactors
(c) all alkali metals give a flame test
(d) lithium is the weakest reducing agent among the alkali metals
Show Answer & Explanation

Answer: (d) lithium is the weakest reducing agent among the alkali metals

Explanation:
1. Lithium has the most negative E (-3.04 V) because of its very high hydration enthalpy, so it is the strongest reducing agent in water.

Teacher's Note:
1. Do not judge reducing power by ionisation energy alone.
2. Hydration enthalpy changes the order.

Question 4: Consider: S1 alkali metals never occur free in nature; S2 their melting and boiling points are high; S3 Cs and K are used as electrodes in photoelectric cells; S4 alkali metals are kept in kerosene. The truth values in order are
(a) T T F F
(b) T F T T
(c) F F F T
(d) T T F T
Show Answer & Explanation

Answer: (b) T F T T

Explanation:
1. S1 true (very reactive). S2 false (they are low, 454 K for Li to 302 K for Cs). S3 true (low ionisation energy). S4 true.

Teacher's Note:
1. Alkali metals melt at low temperatures.
2. Li is sometimes stored in paraffin because it floats on kerosene.

Question 5: Which of the following reacts with water at room temperature?
(a) Be
(b) Li
(c) Mg
(d) all of these
Show Answer & Explanation

Answer: (b) Li

Explanation:
1. Li reacts slowly with cold water. Be does not react and Mg reacts only with boiling water.

Teacher's Note:
1. Order of reactivity: alkali metals, then Ca, Sr, Ba.
2. Mg reacts with steam.

Question 6: Which of the following does not react significantly with air at room temperature?
(a) Li
(b) Ba
(c) all of these
(d) Be
Show Answer & Explanation

Answer: (d) Be

Explanation:
1. Be (and Mg) are protected by a thin oxide layer. Li and Ba are attacked by air.

Teacher's Note:
1. The oxide film is non-porous.
2. Powdered Be burns brilliantly on heating.

Question 7: The pair of amphoteric hydroxides is
(a) \( \ce{Be(OH)2, Al(OH)3} \)
(b) \( \ce{Al(OH)3, LiOH} \)
(c) \( \ce{B(OH)3, Be(OH)2} \)
(d) \( \ce{Be(OH)2, Mg(OH)2} \)
Show Answer & Explanation

Answer: (a) \( \ce{Be(OH)2, Al(OH)3} \)

Explanation:
1. Be(OH)2 and Al(OH)3 both dissolve in acid and alkali. LiOH and Mg(OH)2 are basic, B(OH)3 is a weak acid.

Teacher's Note:
1. Diagonal relationship between Be and Al.
2. Mg(OH)2 does not dissolve in NaOH.

Question 8: The drying agent that reacts with CO\(_2\) and also removes water vapour from ammonia is
(a) \( \ce{CaCl2} \)
(b) \( \ce{CaCO3} \)
(c) \( \ce{CaO} \)
(d) \( \ce{Ca(NO3)2} \)
Show Answer & Explanation

Answer: (c) \( \ce{CaO} \)

Explanation:
1. CaO absorbs moisture and CO2 and is basic, so it does not react with NH3. CaCl2 forms an adduct with ammonia.

Teacher's Note:
1. Quicklime is the standard drying agent for NH3.
2. CaCl2 cannot be used for ammonia.

Question 9: Brine on electrolysis will not give
(a) \( \ce{NaOH} \)
(b) \( \ce{Cl2} \)
(c) \( \ce{H2} \)
(d) \( \ce{CO2} \)
Show Answer & Explanation

Answer: (d) \( \ce{CO2} \)

Explanation:
1. \( \ce{2NaCl + 2H2O -> 2NaOH + Cl2 + H2} \). No carbon source is present.

Teacher's Note:
1. Chlor-alkali process products.
2. Check the elements present.

Question 10: Chemical A removes temporary hardness, reacts with Na\(_2\)CO\(_3\) to give caustic soda, and turns milky with CO\(_2\). A is
(a) \( \ce{CaCO3} \)
(b) \( \ce{Ca(OH)2} \)
(c) \( \ce{CaO} \)
(d) \( \ce{Ca(HCO3)2} \)
Show Answer & Explanation

Answer: (b) \( \ce{Ca(OH)2} \)

Explanation:
1. \( \ce{Ca(OH)2 + Na2CO3 -> CaCO3 + 2NaOH} \) and \( \ce{Ca(OH)2 + CO2 -> CaCO3 + H2O} \) (milky). Clark's method uses slaked lime.

Teacher's Note:
1. CaO would also be a candidate in water, but the formula asked is that of the dissolved hydroxide.
2. Lime water turns milky with CO2.

Question 11: A colourless solid X on heating evolves CO\(_2\) and leaves a white residue that dissolves in water and also gives CO\(_2\) with dilute acid. X is
(a) \( \ce{KHCO3} \)
(b) \( \ce{K2CO3} \)
(c) \( \ce{CaCO3} \)
(d) \( \ce{Na2CO3} \)
Show Answer & Explanation

Answer: (a) \( \ce{KHCO3} \)

Explanation:
1. \( \ce{2KHCO3 -> K2CO3 + H2O + CO2} \). K2CO3 is water soluble and evolves CO2 with acid. K2CO3 and Na2CO3 do not evolve CO2 on heating, CaCO3 leaves insoluble CaO.

Teacher's Note:
1. Only a bicarbonate fits.
2. Check the solubility of the residue.

Question 12: Crude common salt becomes damp in air because
(a) NaCl is hygroscopic
(b) NaCl is deliquescent
(c) it contains MgCl\(_2\) and CaCl\(_2\) which are deliquescent
(d) it contains NaOH
Show Answer & Explanation

Answer: (c) it contains MgCl\(_2\) and CaCl\(_2\) which are deliquescent

Explanation:
1. Pure NaCl does not absorb moisture; the impurities do.

Teacher's Note:
1. Pure table salt is purified by saturating with HCl gas.
2. Deliquescent salts dissolve in absorbed water.

Question 13: Calcium cyanamide on reaction with steam under pressure gives ammonia and
(a) \( \ce{Ca(OH)2} \)
(b) \( \ce{CaO} \)
(c) \( \ce{CaCO3} \)
(d) \( \ce{Ca(HCO3)2} \)
Show Answer & Explanation

Answer: (c) \( \ce{CaCO3} \)

Explanation:
1. \( \ce{CaNCN + 3H2O -> CaCO3 + 2NH3} \).

Teacher's Note:
1. Cyanamide is hydrolysed completely.
2. Used to make ammonia from the fertiliser.

Question 14: How many of the following flame colours are correctly matched? K violet, Na yellow, Be crimson red, Ca brick red, Sr apple green, Mg none, Rb red violet, Cs blue, Li crimson red.
(a) 7
(b) 5
(c) 6
(d) 8
Show Answer & Explanation

Answer: (a) 7

Explanation:
1. Be gives no colour and Sr gives crimson red, so those two are wrong; the other seven are correct.

Teacher's Note:
1. Ba gives apple green.
2. Count carefully.

Question 15: How many of BeCl\(_2\), AlCl\(_3\), NaHCO\(_3\), Li\(_2\)CO\(_3\), BeH\(_2\), Na\(_2\)CO\(_3\) form polymeric chains in the solid state?
(a) 2
(b) 4
(c) 5
(d) 3
Show Answer & Explanation

Answer: (d) 3

Explanation:
1. BeCl2 (chain), NaHCO3 (hydrogen-bonded chains) and BeH2 (hydrogen-bridged chain). AlCl3 is a dimer or layer lattice, the carbonates are ionic.

Teacher's Note:
1. Chain structures arise from bridging or hydrogen bonds.
2. Do not count dimers.

Question 16: For an alkali metal M: M\(_2\)O + H\(_2\)O gives x; M\(_2\)O\(_2\) + H\(_2\)O gives x + y; MO\(_3\) + H\(_2\)O gives x + y + z. The total number of atoms in one molecule each of x, y and z is
(a) 7
(b) 9
(c) 8
(d) 10
Show Answer & Explanation

Answer: (b) 9

Explanation:
1. x = MOH (3 atoms), y = H2O2 (4 atoms), z = O2 (2 atoms). Total 9.

Teacher's Note:
1. The peroxide gives H2O2 and the ozonide also gives oxygen.
2. Add atoms per formula unit.

Question 17: NaOH + PbO gives x; NaOH + SnO\(_2\) gives y; NaOH + H\(_2\)O + Al gives z + H\(_2\) (assume no complex formation). The total number of atoms in x, y and z is
(a) 12
(b) 15
(c) 14
(d) 16
Show Answer & Explanation

Answer: (b) 15

Explanation:
1. x = Na2PbO2 (5), y = Na2SnO3 (6), z = NaAlO2 (4). Total 15.

Teacher's Note:
1. These oxides are amphoteric.
2. Aluminium liberates hydrogen from alkali.

Question 18: Out of Li\(_2\)O, KO\(_3\), RbO\(_2\), Cs\(_2\)O\(_2\), BeO, MgO, BaO\(_2\), SrO, how many turn moist red litmus blue and finally white?
(a) 3
(b) 5
(c) 6
(d) 4
Show Answer & Explanation

Answer: (d) 4

Explanation:
1. Bleaching needs H2O2 generated on hydrolysis: KO3, RbO2, Cs2O2 and BaO2. The normal oxides only give a blue colour.

Teacher's Note:
1. Peroxides and superoxides bleach through H2O2.
2. Li2O, SrO give bases only; BeO, MgO are insoluble.

Question 19: In the Solvay process the by-product reacts with Na\(_2\)CO\(_3\) to give a compound x, which on heating gives y. The number of atoms in one molecule of y is
(a) 3
(b) 2
(c) 4
(d) 5
Show Answer & Explanation

Answer: (a) 3

Explanation:
1. The by-product CaCl2 reacts with Na2CO3 to give CaCO3 (x), which decomposes to CaO + CO2 (y). CO2 has 3 atoms.

Teacher's Note:
1. CO2 is absorbed by KO2.
2. Follow the chain step by step.

Question 20: How many of these statements are correct? (a) Solvay process makes sodium carbonate. (b) CaCl\(_2\) is a by-product. (c) NH\(_3\) is recovered. (d) MgSO\(_4\).7H\(_2\)O is Epsom salt. (e) Hydrolysis of Na\(_2\)CO\(_3\) gives an acidic solution. (f) K\(_2\)CO\(_3\) can also be made by the Solvay process. (g) Excess CO\(_2\) through lime water gives CaCO\(_3\).
(a) 3
(b) 5
(c) 4
(d) 6
Show Answer & Explanation

Answer: (c) 4

Explanation:
1. (a)-(d) are correct. (e) is wrong, the solution is basic. (f) is wrong. (g) is wrong, excess CO2 gives soluble Ca(HCO3)2.

Teacher's Note:
1. Only the first four are right.
2. Check each statement separately.

Question 21: In the Solvay process A + B + H\(_2\)O gives NH\(_4\)HCO\(_3\); NH\(_4\)HCO\(_3\) + NaCl gives C + NH\(_4\)Cl; NH\(_4\)Cl + D gives 2NH\(_3\) + 2H\(_2\)O + CaCl\(_2\). The total number of atoms in one molecule each of A, B, C and D is
(a) 15
(b) 16
(c) 20
(d) 18
Show Answer & Explanation

Answer: (d) 18

Explanation:
1. A = NH3 (4), B = CO2 (3), C = NaHCO3 (6), D = Ca(OH)2 (5). Total 18.

Teacher's Note:
1. D is slaked lime.
2. Balance the third equation to identify D.

Question 22: Glauber's salt is Na\(_2\)SO\(_4\).xH\(_2\)O. The value of x is
(a) 7
(b) 10
(c) 5
(d) 2
Show Answer & Explanation

Answer: (b) 10

Explanation:
1. Glauber's salt is sodium sulphate decahydrate.

Teacher's Note:
1. Epsom salt has 7 water molecules.
2. Washing soda also has 10.

Question 23: Which of these are NOT correct configurations of s-block elements? (1) [Ar]3d\(^{10}\)4s\(^2\) (2) [Ar]3d\(^{10}\)4s\(^1\) (3) [Ar]4s\(^2\) (4) [Ar]4s\(^1\)
(a) (1) and (2)
(b) (3) and (4)
(c) (1) only
(d) (2) and (4)
Show Answer & Explanation

Answer: (a) (1) and (2)

Explanation:
1. (1) is Zn and (2) is Cu, both d-block. (3) is Ca and (4) is K.

Teacher's Note:
1. An s-block element has no filled d-subshell beyond the noble gas core.
2. Check the atomic number.

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