Deduce Alkaline Buffer on Mixing 2 Solutions
A buffer is a solution that maintains pH when small amounts of H+ or OH- is added to it.
It is essentially a mixture of conjugate acid-base pair:
a. acidic buffer = weak acid + conjugate base
b. alkaline buffer = weak base + conjugate acid
The acid will remove OH- added while the base partner will remove H+ added, thereby maintaining pH.
For this question we need to determine the resultant solution when 2 solutions are mixed.
A useful way to determine resultant solution is using the ICE table when there is an acid-base reaction.
A. 10 cm3 of 0.1 mol dm-3 NaOH and 20 cm3 of 0.2 mol dm-3 NH4+Cl-
NaOH is a strong base while NH4+ is conjugate acid of weak base NH3.
Resultant solution is 0.001 mol of weak base NH3 + 0.003 mol conjugate acid NH4+.
This is an alkaline buffer and can be our answer but let's go through the other options first.
B. 25 cm3 of 0.1 mol dm-3 NaOH and 50 cm3 of 0.1 mol dm-3 CH3CO2-Na+
NaOH is a strong base while CH3CO2- is conjugate base of weak acid CH3COOH.
Resultant solution is a mixture of 2 bases which cannot function as a buffer.
C. 10 cm3 of 1 mol dm-3 H2SO4 and 20 cm3 of 1 mol dm-3 CH3CH2NH2
H2SO4 is a strong acid while CH3CH2NH2 is a weak base.
Resultant solution is 0.02 mol of conjugate acid CH3CH2NH3+ which is not a buffer.
D. 50 cm3 of 0.05 mol dm-3 NaOH to 50 cm3 of 0.10 mol dm-3 CH3NH3+Cl-
NaOH is a strong base while CH3NH3+ is conjugate acid of weak base CH3NH2.
Resultant solution is 0.0025 mol of weak base CH3NH2 + 0.0025 mol of conjugate acid CH3NH3+.
We have another alkaline buffer and the moles of weak base and conjugate acid are exactly the same.
This buffer is at its maximum buffering capacity so pH will be a constant and can best resist changes to pH.
Therefore answer to this question is option D.
Topic: Buffer Solution, Physical Chemistry, A Level Chemistry, Singapore
Please SUBSCRIBE to my channel and SHARE this video with your friends if you find it helpful!
View the full video with description at my website https://chemistryguru.com.sg/det-alka...
Looking for more FREE A Level Chemistry videos? Check out our huge collection of video lessons at https://chemistryguru.com.sg/a-level-...