Question

equal volumes of solutions of the given concentrations are mixed together. which of the resulting aqueous solutions makes a buffer?

Answer 1

a buffer solution is defined as a solution which resists any change in its pH value(i.e. whose pH remains practically constant )even when small amounts of acid or base are added to it..

there are two types of buffer solutions:
(1) solution of single substances --> the solution of a weak acid and a weak base.. for example ammonium acetate (CH3COONH4) act as a buffer..
(2)solution of mixtures--> these are furthur of two types..
A) acidic buffer ---> it is the solution of weak acid and a salt of this weak acid with a strong base ..for example--CH3COOH + CH3COONa....here CH3COOH is a weak acid and CH3COONa is a salt of weak acid (CH3COOH) and strong base(NaOH)
B)basic buffer --->it is the solution of a mixture of weak acid and a salt of this weak base with a strong acid ...for example - NH4OH + NH4Cl ...here NH4OH is a weak base and NH4Cl is a salt of weak base (NH4OH) and strong acid (HCl)..

first one is wrong because HCl is a strong acid and NaOH is a strong base ...so they cant form a buffer solution ..

fourth one is wrong because HCl is a strong acid and CH3COOH is a weak acid ...so no buffer...

fifth one is wrong because HCl is a strong acid and NaCl is neutral because its a salt of strong acid (HCl and strong base (NaOH) ..so the solution is mostly acidic and cannot neutralize any added acid acid so no buffer ability...

second and third one can act as buffer because NH3 is a weak base and when HCl is added to it the following reaction takes place :
NH3 + HCl ----> NH4Cl i.e. a salt of weak base and strong acid...
so NH3 and HCl mixture can act as a basic buffer..
now when 0.10M HCl is added to 0.05M NH3 ..as HCl concentration is more than NH3 so it will consume the entire NH3,according to the given reaction and now the mixture will have NH4Cl and HCl ...so the solution contains strong acid and salt of weak base and strong acid so it cant act as a buffer according to the definition...

when 0.10 M HCl is added to 0.15 M NH3 ...as NH3 concentration is more than HCl so it will not be entirely consumed ... NH4Cl will be present and NH3 will be present so as the solution contains weak base and a salt of weak base and strong acid so it can act as a buffer solution...