Question:

The pH of a 0.1 M aqueous solution of $NH_4OH$ is : $(K_b = 1.0 \times 10^{-5})$

Updated On: Jul 7, 2022
  • 3
  • 10.5
  • 11
  • 7.5
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The Correct Option is C

Solution and Explanation

$\begin{matrix}&NH_{4}OH& {<=>}&NH^{+}_{4}&+&OH^{-}\\ \text{Initial }&0.1&&0&&0\\ \text{At eqm. }&0.1(1-\alpha)&&0.1\alpha&&0.1 \alpha\end{matrix}$ $\therefore K_{b}=\frac{0.1\alpha\times0.1\alpha}{0.1}=0.1\,\alpha^{2}$ $0.1 \alpha^{2}=1\times10^{-5}$ $\alpha^{2}=10^{-4} \Rightarrow \alpha=10^{-2}$ $\left[OH^{-}\right]=c\alpha=0.1\times10^{-2}=10^{-3}M$ $\left[H^{+}\right]=10^{-14}10^{-3}=10^{-11}$ $\therefore pH=11$
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Notes on Acids and Bases

Concepts Used:

Acids and Bases

Acid is any hydrogen-containing substance that is capable of donating a proton (hydrogen ion) to another substance. Base is an ion or molecule capable of accepting a hydrogen ion from acid. 

Physical Properties of Acids and Bases

Physical PropertiesACIDSBASES
TasteSourBitter
Colour on Litmus paper Turns blue litmus redTurns red litmus blue
Ions produced on dissociationH+OH-
pH<7 (less than 7)>7 (more than 7)
Strong acidsHCl, HNO3, H2SO4NaOH, KOH
Weak AcidsCH3COOH, H3PO4, H2CO3NH4OH

Chemical Properties of Acids and Bases

Type of ReactionAcidBases
Reaction with Metals

Acid + Metal → Salt + Hydrogen gas (H2)

E.g.,

Zn(s)+ dil. H2SO4 → ZnSO4 (Zinc Sulphate) + H2

Base + Metal → Salt + Hydrogen gas (H2)

E.g.,

2NaOH +Zn → Na2ZnO2 (Sodium zincate) + H2

Reaction with hydrogen carbonates (bicarbonate) and carbonates

Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water

E.g., HCl+NaOH → NaCl+ H2O

2. Na2CO3+ 2 HCl(aq) →2NaCl(aq)+ H2O(l) + CO2(g)

3. Na2CO3+ 2H2SO4(aq) →2Na2SO4(aq)+ H2O(l) + CO2(g)

4. NaHCO3+ HCl → NaCl+ H2O+ CO2

Base+ Carbonate/ bicarbonate → No reaction
Neutralisation Reaction

Base + Acid → Salt + Water

E.g., NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Base + Acid → Salt + Water

E.g., CaO+ HCl (l) → CaCl2 (aq)+ H2O (l)

Reaction with Oxides

Metal oxide + Acid → Salt + Water

E.g., CaO+ HCl (l) → CaCl2 (aq)+ H2O (l)

Non- Metallic oxide + Base → Salt + Water

E.g., Ca(OH)2+ CO2 → CaCO3+ H2O

Dissolution in Water

Acid gives H+ ions in water.

E.g., HCl → H+ + Cl-

HCl + H2O → H3O+ + Cl–

Base gives OH- ions in water.

Read more on Acids, Bases and Salts