Question

D Question 19 3 pts Which one of the following additions will react with 25.0 mL of 0.30 M NHto form a buffer? O 30.0 mL of 0.20 M Naci O 37.5 mL of 0.20 M HCI 30.0 mL of 0.20 M HCI 0 40.0 mL of 0.20 M NaCl

Answer 1

A buffer is a solution which resist the change in pH when small amount of either an acid or a base is added and it consisting of either a weak acid and its salt or a weak base and its salt.

NH₃ is a weak base so a buffer will be formed when it is with its salt like NH₄Cl. NaCl is salt but it is not a salt of ammonia. From the definition of a buffer, a buffer must be formed from a weak base and its salt. So when ammonia is added to a solution of HCl it will form NH₄Cl. But there must some unreacted base to be the solution to a buffer solution.

When 37.5 mL of 0.20M HCl is added in 25.0 mL 0.3M NH₃ it will completely react with NH₃ to form NH₄Cl.

Because 37.5 mL of 0.20M HCl ≡ 25.0 mL 0.3M NH₃ (this can be shown from N1V1=N2V2)i.e. there will be no unreacted NH₃ present to the solution.

But when 30 mL of 0.20M HCl is added in 25.0 mL 0.3M NH₃ it will HCl will completely react with NH₃ to form NH₄Cl but since the amount of HCl is less compared to ammonia so there will be some unreacted ammonia present in the solution so it will form a buffer solution of a weak base NH3 and its salt NH₄Cl.

Because 30 mL of 0.20M HCl ≡ 20.0 mL 0.3M NH₃

So when a small amount of an acid is added to the buffer it will react with unreacted ammonia(NH₃) it will form ammonium ion and reduces the H⁺ ion concentration.

NH₃ + H⁺ -----> NH₄⁺

And when a small amount of OH^- it will react with NH₄⁺ to form NH₃ and water and reduces the OH^- ion concentration.

NH₄⁺ + OH^- -----> NH₃ + H₂O

i.e. in both the cases pH remains unaltered.