Question

0.1 mole of CH₃NH₂ (K_b = 5 × 10⁻⁴) is mixed with 0.08 mole of HCl and diluted to one litre. What will be the H⁺ concentration in the solution?

• A. 8 × 10⁻² M
• B. 8 × 10⁻¹¹ M
• C. 1.6 × 10⁻¹¹ M
• D. 8 × 10⁻⁵ M

Answer 1

Answer: (B)

8 × 10⁻¹¹ M

Answer 2

CH₃NH₂ + HCl $\mathrm { CH } _ { 3 } \mathrm { NH } _ { 3 } ^ { + } \mathrm { Cl } ^ { - }$

0.1 0.08 0

0.02 0 0.08

(Basic buffer solution)

pOH = pK_b + log $\frac { 0.08 } { 0.02 }$

= pK_b + 0.602

= 3.30 + 0.602= 3.902

∴ pH = 10.09

[H⁺] = 7.99 × 10⁻¹¹ ≈ 8 × 10⁻¹¹ M