Which will make basic buffer ? 100 mL of 0.1M HCl + 200 mL of 0.1M NH4OH

Question : Which will make basic buffer ?

(1) 50 mL of 0.1M NaOH + 25 mL of 0.1 M CH₃COOH

(2) 100 mL of 0.1M CH₃COOH + 100 mL of 0.1M NaOH

(3) 100 mL of 0.1M HCl + 200 mL of 0.1M NH₄OH

(4) 100 mL of 0.1M Hcl + 100 mL of 0.1M NaOH

answer : from all above options the correct answer is “(3) 100 mL of 0.1M HCl + 200 mL of 0.1M NH₄OH”

Explanation of correct answer :

Out of the given options , NaOH is a strong base , so it’s conjugate acid is a weak. Similarly , HCl is a strong acid so it’s conjugate base is weaker. CH₃COOH is a weak acid , so its conjugate base is stronger. NH₄OH is a weak base , so its conjugate acid is stronger.

NaOH + CH₃COOH → CH₃COONa + H₂O

NaOH + HCl → NaCl + H₂O

NH₄OH + HCl → NH₄Cl + H₂O

So , 1 mol of NaOH reacts with 1 mol of CH₃COOH

For (1) : 50/1000  x 0.1 mol of NaOH reacts with

25/1000  x 0.1  M of CH₄COOH

As number of moles of CH₃COOH is less , it is wholly consumed in the reaction. So , unreacted NaOH = 0.005 – 0.0025 = 0.0025 mol

For salt hydrolysis of weak acid (CH₃COOH) and strong base (NaOH) , we have

pH = ½ [pK_w + pK_a + log [CH₃COO^–]]

V_total = 50 + 25 = 75 mL.

[CH₃COO^–] = 0.0025 x 75 / 1000 = 0.0001875 M

So

pH = 7 + pK_a/2 + log(0.0001875) = 7 + pK_a/2  – 3.72 = 3.27 + pK_a/2

So , it forms an acidic buffer.

For option (2) : 100/1000  x 0.1 mol of NaOH reacts with

100/1000  x 0.1 M of CH₃COOH

As number of moles of NaOH and CH₃COOH are equal , both are wholly consumed in this reaction.

For salt hydrolysis of weak acid (CH₃COOH) and strong base (NaOH) , we have

pH = ½ [pK_w + pK_a + log [CH₃COO^–]]

V_total = 100 + 100 = 200 mL

[CH₃COO^–] = 0.01 x 200 / 1000 = 0.002M

So , pH = 7 + pK_a/2 + log (0.002) = 7 + pK_a/2 – 2.69 = 4.3 + pK_a/2

So , it forms and acidic buffer.

For option (3) 200/1000 x 0.1 mol of NH₄OH reacts with

100/1000 x 0.1 M of HCl

As number of moles of HCl is less , it is wholly consumed in the reaction. So unreacted NH₄OH = 0.02 – 0.01 = 0.01 mol

For salt hydrolysis of strong acid (HCl) and weak base (NH₄OH) , we have

pH = ½ [pK_w + pK_b – log [NH₄Cl]]

V_total = 200 + 100 = 300 ml

[NH₄Cl]  = 0.01 x 300 / 1000 = 0.003 M

So , pH = 7 + pK_b/2 – log (0.003) = 7 + pK_b/2 + 2.69 = 9.69 + pK_b/2

So , it forms a basic buffer.

For option (4) : 100/1000  x 0.1 mol of NaOH reacts with

100/1000 x 0.01 M of HCl

As number of moles of HCl and NaOH are equal , they completely reacts with each other and no buffer is formed.

extra information : buffer is a type of solution that contains acidity or we can say alkalinity and this type of solution will oppose the change of pH when some acid or alkali is mixed with it in small amount.

option (1) : this is an example of basics solution not an example of basic buffer solution. so this is incorrect answer.

option (2) : in this reaction Hydrolysis of salt happened but not basic buffer solution so this is also incorrect answer.