Question

8. Which one of the following substances is not a base? a) KOH b) Ca(ОН)2 c) NH3 d) HNO3 e) Cl 9. Which of the following species is NOT soluble? (a) Na2S (b) Рb(ОН)2 (c) KNO3 (d) KCl (e) (NH4)2CO3 10. Which of the following is a weak acid? a) HBr b) HCl c) NH3 d) HCIO4 e) HF

Answer 1

8.

A base is a compound that produces excess of hydroxide ions in solution.

Lets check which of the options are base and find the odd one out.

a) KOH

It is a strong base that completely dissociates to form hydroxide ions and potassium cations.

$KOH_{(aq)} \rightarrow K^+_{(aq)} + OH^-_{(aq)}$

b) $Ca(OH)_2$

It is also a strong base that dissociates into Ca2+ and OH- ions.

  $Ca(OH)_2_{(aq)} \rightarrow Ca^{2+}_{(aq)} + OH^-_{(aq)}$

c) $NH_3$

It does not directly dissociate to produce OH- ions, but its reaction with water forms OH- ion as it abstracts a proton from water molecule.

Hence, it is also a weak base.

$NH_3_{(aq)} + H_2O_{(l)} \rightarrow NH_4^+_{(aq)} + OH^-_{(aq)}$

d) $HNO_3$

It produces H3O+ in its reaction with water. Hence, it is an acid.

$HNO_3_{(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + NO_3^-_{(aq)}$

Hence, it is the correct choice.

e) $Cl^-$

It is the conjugate base of hydrochloric acid HCl. Hence, it can also abstract proton to form OH- in solution.

$Cl^-_{(aq)} + H_2O_{(l)} \rightarrow HCl_{(aq)} + OH^-_{(aq)}$

Hence, it is also a base.

Hence, the correct choice is d) $HNO_3$

9.

Lets check the solubility of each of the given compounds using the solubility rules

a) $Na_2S$

Salt containing group I ions like Na+, Li+, K+ are soluble. Hence, since Na2S contains Na+ it is soluble.

b)$Pb(OH)_2$

Most hydroxides are insoluble and form precipitates in water. Also Pb2+ cation forms very insoluble salts. Hence, it is insoluble.

c) $KNO_3$

Salt containing group I cations are soluble. Also salts containing NO3- are soluble. Hence, KNO3 is very soluble in water.

d)$KCl$

KCl is very much soluble since it contains a group I cation. Also salts containing Cl- are very soluble.

e)$(NH_4)_2CO_3$

Carbonates are frequently insoluble. But presence of NH4+ as cation makes it soluble in water.

Hence, the correct choice is b) $Pb(OH)_2$.

10.

There are seven acids that are considered strong as they dissociate completely in water. They are HCl, HBr, HI, HNO3, HClO4, H2SO4, HClO3.

Other acids except these are considered weak acid.

Hence, option a), b) and d) are all strong acids.

Option c) NH3 is not an acid but a weak base.

Now, option e) HF is a weak acid because of the high electronegativity of F atom which binds the H tightly. Because of this strong H-F bond, protonation is not very easy.

Hence, the correct choice is e) HF.