3. An aqueous solution contains 0.01M RNH₂ ( Kb = 2x10-6 ) & 10-4 M NaOH. The concentration of OH- is nearly ?

3. An aqueous solution contains 0.01M RNH₂ ( Kb = 2x10^-6 ) & 10^-4 M NaOH. The concentration of OH^- is nearly ?

A. 2.414×10^-4 

B. 2×10^-4 

C. 1.414×10^-5

D. 10^-4 

E. 1.414×10^-4 

SOLUTION

[OH−] ⟹ RNH2​ is weak base, it reacts with H2​O

pKb​ = −log1​0 (2×10^-6) 

= 6 − log2

pH = 14 − 21​ (pKb​ − logC)

pH = 14 − 21​ (6-log2 − log10−2)

pH = 14 − 3 + 2 log2​ + 21​(−2)

pH = 10 + 2 log2​

pH = 10.150515

NaOH ⟹ [OH−]= 10^-4 M

pOH = 14 − pH

pOH = 0.0001414 ⟹ 1.414×10^-4 M

So, the correct concentration of OH^- is 1.414×10^-4 M (E)