Acids and Bases Questions and Answers

Acids and bases questions answers

  • Problem
    What is the acidic order of oxoacids of phosphorus?
  • Answer
    When the oxidation state rule applied to the oxoacids of phosphorus (H3PO2, H3PO3 and H3PO4) it is predicted that the acidic Character of these acids should be in the order:
    H3PO2H3PO3H3PO4
    But the experimental observation suggested the reverse order:
    H3PO2H3PO3H3PO4
    The experimental order can be explained when we consider the structures of these acids given as,
Questions and its Answers of Acids and Bases
Structure of oxoacids of Phosphorus.
    H₃PO₂ is a monobasic acid. The proton attached to oxygen has a far greater chance of dissociation than any directly bonded hydrogen.
    The structure of H₃PO₂ involves one protonated oxygen and another unprotonated oxygen. H₃PO₃ is dibasic and hence has two protonated oxygens and one unprotonated oxygen. H₃PO₄ is tribasic, has three protonated oxygens and one unprotonated oxygen.
    In this series, therefore, the number of unprotonated oxygen's, which are the vehicles for the enhancement of acidity, is the same for all the three acids. But dissociable protons increase from one in H₃PO₂ to three in H₃PO₄.
    Therefore the overall inductive effect of the unprotonated oxygen decreases from H₃PO₂ to H₃PO₄. Hence the acidity slightly falls off in the order:
    H3PO2H3PO3H3PO4
  • Problem
    What will be the effect of adding KNH2 to liquid ammonia with respect to acidity?
  • Answer
2NH3 NH4+ + NH2-
KNH2 K+ + NH2-
    Due to common ion effect the equilibrium shift to left and decrees the acidity of NH3.
  • Problem
    Why all alkali are based but all bases are not alkali?
  • Answer
    All the bases are dissolved in water to produce alkali whereas all the bases are not dissolved in water but all the alkali are dissolved in water. Thus all alkali are based but all bases are not alkali. Na₂O is a base because it dissociates in water to produce NaOH.
    NaOH, KOH and Ca(OH)₂ dissolved in water to produce OH⁻ ion. Thus all these hydroxides are Alkali.
NaOH Na+ + OH-
KOH K+ + OH-
Ca(OH)2 Ca+2 + 2OH-
    Al(OH)3, Fe(OH)3, Zn(OH)2 etc does not dissolve in water but reacts with acids to produce Salt and water. Thus These are bases but not alkali.
  • Problem
    Arrange these oxides in order of their acidic nature: N2O5, As2O3, Na2O, MgO.
  • Answer
    Acidic oxides react with water to give oxoacids. The higher the oxidation number and the higher the electronegativity the greater the central element will force to reaction with water to give the oxoacid. Of all these oxoacids nitrogen has the highest oxidation number and electronegativity. Thus the order of acidic nature and oxidation number are-
Na2O MgO As2O3 N2O5
+1 +2 +3 +5
  • Problem
  • Answer
We have from the definition,
pH = - log[H⁺] = 4.5
or, log[H⁺] = - 4.5
∴ [H⁺] = 3.16 × 10⁻⁵
  • Problem
    Calculate the [H+], [OH-] and pH of a solution prepared by diluting 20 ml of 0.1M HCl to one lit.
  • Answer
    [H⁺] = (20×0.1)/1000 = 0.002 = 2×10⁻³
    [OH⁻] = (1×10⁻¹⁴)/[H⁺] = (1×10⁻¹⁴)/(2×10⁻³) = 0.5×10⁻¹¹
    pH = -log[H⁺] = -log2×10⁻³ = 3-log2 = 2.7
  • Problem
    Calculate the [H+], [OH-] and pH of a solution obtained by dissolving 28 gm KOH to make 200 ml of a solution.
  • Answer
    [OH⁻] = (28×1000)/(56×200) = 2.5M (Molecular Weight of KOH = 56gm)
    [H⁺] = (1×10⁻¹⁴)/[OH⁻] = (1×10⁻¹⁴)/(2.5) = 4×10⁻¹⁵
    pH = -log[H⁺] = - log4 × 10⁻¹⁵ = (15 - log4)
  • Problem
    Name the conjugate acids and the conjugate bases of HX-1 and X-2.
  • Answer
    Conjugate acid of a species is the one that is obtained on the addition of a proton and conjugate base of a species is one that is obtained on the release of a proton.
H2O + HX- OH- + H2X
Acid1
Base2 Base1 Acid2
    In the above reaction HX- acts as a base and its conjugate acid is H2X.
HX- + H2O X-2 + H3O+
Acid1
Base2 Base1 Acid2
    In this reaction HX- acts as an acid thus its conjugated base is X-2. In the same way, the conjugate acid of X-2 is HX- but X-2 cannot have any conjugated base because there is no proton that can release.
  • Problem
    Bisulphate ion can be viewed both as an acid and a base. Explain.
  • Answer
    Bisulphate ion is HSO3-. It may lose a proton to give the conjugate base SO3-2, thus behaving as an acid. Again it may add on a proton to give the conjugate acid, thus showing its base character.
  • Problem
    Arrange in order of Lewis acidity : (i) BF3 (ii) BCl3 (iii) BI3 (iv) BBr3
  • Answer
    These boron halides have pi-interaction between filled p- orbitals of the halogen and empty p-orbital of boron. The effectiveness of the pi-interaction falls off with the increasing size of halogens so that it is the strongest in BI3. When boron halides receive an electron pair the pi-bond between boron and halogen has to be ruptured in order to make room for a coordinate bond.
    H3NBF3
    Thus with BF3, it will be hardest to rupture the pi-bond. Therefore the order of the Lewis acidity is: 

BF₃ㄑBCl₃ㄑBBr₃ㄑBI₃

  • Problem

    Explain - NH3 behaves as a base but BF3 as an Acid.

  • Answer

    In NH3, the central N atom has lone pair of electrons This lone pair coordinate to empty orbital, is termed as a base according to Lewis concept. The compounds with less than an octet for the central atom are Lewis acids. In BF3, B in BF3 contains six electrons in the central atom thus it behaves as an acid.

    F₃B + :NH₃ ⇆ F₃B ← :NH₃

  • Problem

    Explain why tri-covalent phosphorus compounds can serve both as Lewis acids and also as bases?

  • Answer

    Tri-covalent phosphorus compounds like PCl3 have a lone pair of electrons in phosphorus. This lone pair may coordinate to a metal ion thus allowing the compound to serve as a Lewis base.
Ni + 4PCl3 [Ni(PCl3)4]0

    Again the quantum shell of phosphorus has provision for d-orbitals which can receive back donated electrons from electron reach low oxidation state of a metal ion. In this latter case, tri-covalent phosphorus compound serves as a Lewis acid.

  • Problem

    Bi-positive tin can function both as a Lewis acid and a Lewis base. Explain?

  • Answer

    The Lewis representation of SnCl2 shows a lone pair on tin through it is as yet short of an octet. Ligands, particularly donor solvents with lone pairs, may coordinate to tin giving complexes. Here SnCl2 behaves as a Lewis acid.

    Again interaction of platinum group metal compounds with SnCl2 (SnCl3-) as donor leads to the formation of coordinate complexes. As for examples,
    [(Ph3P)2PtCl(SnCl3)], [RuCl2(SnCl3)2]-2
    These are examples, where SnCl2 acts as a Lewis Base.
  • Problem
    Can SiCl4 and SnCl4 function as Lewis acids?
  • Answer
    Both silicon and tin are members of Group IV of the periodic table and their quantum shells admit of d-orbitals. As a result, they can expand their valence shell through SP³d² hybridisation and can give rise to six-coordinate complexes.
    In fact complex are,
    [SiCl4{N(CH3)3}2], [SiCl2(Py)4]Cl2, [SnCl4(Bpy)]
    Thus SiCl4 and SnCl4 can function as Lewis acids.
  • Problem
    SO3 Behaves as acid and H2O as a base. Explain.
  • Answer
    SO3 like BF3 has less than an octet and will be termed as an acid according to Lewis concept. But in H2O oxygen atom contain lone pairs to donate the Lewis acid.
    Oxygen and sulphur contain six electrons in their valence shell and therefore regarded as Lewis Acids.
    The oxidation of SO3-2 to SO4-2 ion by oxygen and S2O3-2 ion by sulphur are the acid-base reactions.
    SO3-2 + [O] [OS2O3]-2    SO3-2 + [S] [SS2O3]-2
  • Problem
    Explains why Hg(OH)2 dissolved readily in acidic aqueous solution but HgS does not?
  • Answer
    In the case of Hg(OH)2 and HgS, Hg is a soft acid and OH- and S-2 is hard to base and soft base respectively. 
    Evidently, HgS (Soft acid + Soft base) will be more stable than Hg(OH)2 (Soft Acid + Hard base). 
    More stability of HgS than that of Hg(OH)2 explain why Hg(OH)2 dissolved readily in acidic aqueous solution but HgS does not.
  • Problem
    Classify the following as Hard and Soft Acids and Bases. (i) H- (ii) Ni+4 (iii) I+ (iv) H+
  • Answer
  1. The hydride ion has a negative charge and is far too large in size compared to the hydrogen atom. Its electronegativity is quite low and it will be highly polarisable by virtue of its large size. Hence it is Soft Base.
  2. Quadrivalent nickel has quite a high positive charge. Compared to bivalent nickel its size will be much smaller. Its electronegativity will be very high and polarisability will be low. Hence it is Hard Acid.
  3. Mono-positive iodine has a low positive charge and has a large size. It has a low electronegativity and high polarizability. Hence it is Soft Acids.
  4. H+ has the smallest size with a high positive charge density. It has no unshared pair of electrons in its valence shell. All these will give a high electronegativity and very low polarizability. Hence H+ is a Hard Acid.

Arrhenius Concept, Solvent System Concept, Bronsted and Lowery Concept, Lewis Concept and Soft and Hard Acids and Bases Questions and Answers

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