Q.Case Study : The pH scale
Measurement of pH of a solution is very essential as its value should be known when dealing with Biological and Cosmetic application. Hydronium ion concentration in molarity is more conveniently expressed on a logarithmic scale known as the pH scale. For an acidic solution, pH is less than 7 but for basic solution, it is more than 7.
Answer the following questions :

Showing the 12 most recent of 27 on this concept.

1. CBSE 2026Set ANNUAL1 markMCQ
   Q.NaOH(aq), HCl(aq) and NaCl(aq) have concentration of 10^-3 M each. Their pH will be respectively

   (a) 10, 6, 2

   (b) 11, 3, 7

   (c) 10, 3, 7

   (d) 10, 4, 7

   ›Reveal solutionSolution

   NaOH and HCl are both strong electrolytes that fully dissociate, giving [OH-] or [H+] equal to the stated concentration; NaCl is a neutral salt (from a strong acid and strong base) and does not affect pH.

   NaOH (strong base), 10^-3 M:

   Fully dissociates: [OH-] = 10^-3 M

   pOH = -log(10^-3) = 3

   pH = 14 - pOH = 14 - 3 = 11

   HCl (strong acid), 10^-3 M:

   Fully dissociates: [H+] = 10^-3 M

   pH = -log(10^-3) = 3

   …

   🔒 Start your 14-day free trial to see the full solution →
2. CBSE 2026Set ANNUAL1 markMCQ
   Q.The pH value of gastric juice is -

   (a) -3

   (b) -1.2

   (c) -2

   (d) -4

   ›Reveal solutionSolution

   The pH of gastric juice is commonly quoted as approximately 1.2 (highly acidic); of the given options, (b) -1.2 matches this value in magnitude.

   Gastric juice, secreted by the stomach's gastric glands, contains hydrochloric acid (HCl) at a concentration that gives it a strongly acidic pH, essential for activating pepsin and digesting food. NCERT's own table of pH of some common fluids lists gastric juice at pH ≈ 1.2.

   …

   🔒 Start your 14-day free trial to see the full solution →
3. CBSE 2026Set ANNUAL1 markMCQ
   Q.A solution with a pH value less than 7 is:

   (a) Basic

   (b) Acidic

   (c) Saline

   (d) Neutral

   ›Reveal solutionSolution

   pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic, on the standard 0-14 pH scale.

   pH is defined as pH = -log10[H3O+]. A lower pH corresponds to a higher concentration of hydronium (H3O+) ions. Pure water at 25°C has pH = 7 and is neutral. Solutions with pH less than 7 have a higher H+ concentration than pure water and are called acidic; solutions with pH greater than …

   🔒 Start your 14-day free trial to see the full solution →
4. CBSE 2026Set ANNUAL1 markMCQ
   Q.The pH of an aqueous solution is 4.0, the value of its pOH would be

   (a) 4.0

   (b) 6.0

   (c) 8.0

   (d) 10.0

   ›Reveal solutionSolution

   pOH = 14 − pH = 14 − 4 = 10.

   For any aqueous solution at 25°C, the ionic product of water gives pH + pOH = 14. …

   🔒 Start your 14-day free trial to see the full solution →
5. CBSE 2026Set ANNUAL1 mark
   Q.What is the value of Kw at 25 C ?

   ›Reveal solutionSolution

   The ionic product of water Kw at 25 C is 1.0 x 10^-14.

   Water self-ionises: H2O ⇌ H+ + OH-. The ionic product Kw = [H+][OH-]. At 25 C pure water has [H+] = [OH-] = 1.0 x 10^-7 mol L^-1, so:

   Kw = (10^-7)(10^-7) = 1.0 x 10^-14 mol^2 L^-2. …

   🔒 Start your 14-day free trial to see the full solution →
6. CBSE 2025Set ANNUAL1 markMCQ
   Q.With increase in temperature, ionic product of water

   (a) Increases

   (b) Decreases

   (c) Remains unaffected

   (d) Zero

   ›Reveal solutionSolution

   Kw (the ionic product of water) increases with rising temperature.

   Water's self-dissociation (2H2O ⇌ H3O+ + OH-) is an endothermic process. By Le Chatelier's principle, increasing temperature shifts an endothermic equilibrium further towards the products, increasing the concentrations of H3O+ and OH-, a …

   🔒 Start your 14-day free trial to see the full solution →
7. CBSE 2025Set ANNUAL1 markMCQ
   Q.Which of the following has lowest pH value?

   (a) 1M HCl

   (b) 1M NaOH

   (c) 1M H2SO4

   (d) 1M C2H5OH

   ›Reveal solutionSolution

   1M H2SO4 has the lowest pH because, being diprotic, it furnishes roughly twice the [H+] of 1M HCl.

   HCl and H2SO4 are both strong acids, but HCl is monoprotic (1M HCl gives [H+] ≈ 1M, pH = 0) while H2SO4 is diprotic (1M H2SO4 gives [H+] ≈ 2M, since both protons ionise essentially completely, pH = -log(2) ≈ -0.3). NaOH is a strong base (high pH, ~14), and ethanol (C2H5OH) i …

   🔒 Start your 14-day free trial to see the full solution →
8. CBSE 2025Set ANNUAL1 markMCQ
   Q.If 50 ml of 0.1 M HCl and 50 ml of 0.2 M NaOH are mixed then the pH of the resulting solution will be

   (a) 1.30

   (b) 4.2

   (c) 12.70

   (d) 11.70

   ›Reveal solutionSolution

   Mixing 50 mL of 0.1M HCl with 50 mL of 0.2M NaOH leaves excess NaOH, giving pH = 12.70.

   Moles of HCl = 0.050 L x 0.1 mol/L = 0.005 mol.

   Moles of NaOH = 0.050 L x 0.2 mol/L = 0.010 mol.

   HCl + NaOH → NaCl + H2O consumes 0.005 mol of each, leaving excess NaOH = 0.010 - 0.005 = 0.005 mol.

   …

   🔒 Start your 14-day free trial to see the full solution →
9. CBSE 2025Set ANNUAL1 markMCQ
   Q.The value of pH of 0.1 M NaOH solution is equal to

   (a) 13

   (b) 1.2

   (c) 5.0

   (d) 1.0

   ›Reveal solutionSolution

   NaOH is a strong base (100% dissociated); its [OH-] equals its molarity, so pOH = 1 and pH = 14 - pOH = 13.

   Step 1 — Dissociation: NaOH is a strong base, meaning it dissociates completely in water:

   NaOH -> Na+ + OH-

   Since dissociation is 100%, the hydroxide ion concentration equals the given molarity:

   [OH-] = 0.1 M = 10^-1 M

   Step 2 — Calculate pOH:

   pOH = -log10[OH-] = -log10(10^-1) = 1

   …

   🔒 Start your 14-day free trial to see the full solution →
10. CBSE 2025Set ANNUAL1 markMCQ
    Q.If concentration of hydrogen ion [H+] changes by a factor of 100, then the value of pH changes by:

    (a) one unit

    (b) 2 unit

    (c) 10 unit

    (d) 100 unit

    ›Reveal solutionSolution

    Since pH is a logarithmic (base 10) scale, multiplying [H$^+$] by 100 shifts pH by exactly $\log_{10}(100) = 2$ units.

    By definition:

    $pH = -\log_{10}[H^+]$

    Let the initial concentration be $[H^+]_1$ and the new concentration be $[H^+]_2 = 100 \times [H^+]_1$ (changed by a factor of 100).

    $pH_1 = -\log[H^+]_1$

    $pH_2 = -\log(100 \times [H^+]_1) = -\log(100) - \log[H^+]_1 = -2 + pH_1$

    $\Delta pH = pH_1 - pH_2 = 2$

    …

    🔒 Start your 14-day free trial to see the full solution →
11. CBSE 2025Set hz1 markMCQ
    Q.Select the correct one: The hydronium ion concentration i.e. [H3O+] of a solution is 2x10^-5 M, its pH is:

    (a) 5

    (b) 7

    (c) 4.7

    (d) 3

    ›Reveal solutionSolution

    pH = -log[H3O+]; substituting [H3O+] = 2x10^-5 M gives pH = 4.7.

    By definition: pH = -log10[H3O+]

    Given [H3O+] = 2 x 10^-5 M.

    pH = -log(2 x 10^-5)

    = -[log 2 + log 10^-5] …

    🔒 Start your 14-day free trial to see the full solution →
12. CBSE 2024Set ANNUAL1 markMCQ
    Q.The pH of 10^-4 M NaOH solution is

    (a) 4

    (b) 10

    (c) 12

    (d) 2

    ›Reveal solutionSolution

    10⁻⁴ M NaOH has pOH = 4, so pH = 10.

    NaOH is a strong base, fully dissociated, so $[OH^-] = 10^{-4}$ M.

    …

    🔒 Start your 14-day free trial to see the full solution →