Question

100 mL of each of the following solutions is mixed; which one of the mixed solutions is a buffer?

A) 1.0 M NH3(aq) + 0.6 M KOH(aq)
B) 1.0 M NH4Cl(aq) + 1.0 M KOH(aq)
C) 1.0 M NH3(aq) + 0.4 M HCl(aq)
D) 1.0 M NH4Cl(aq) + 0.4 M HCl(aq)
E) 1.0 M NH3(aq) + 1.0 M HCl(aq)

I am unsure how to start this problem?

Answer 1

Look at C.
NH3 + HCl ==> NH4Cl + H2O

initial moles:
NH3 = 1.0 M x 100 mL (0.1 L) = 0.1 mole
HCl = 0.4 M x 100 mL (0.1 L) = 0.04 mole
NH4Cl and water = 0

Since this is an acid base system and since HCl is a strong acid, it will react completely.
Final moles:
NH4Cl = 0.0.04
HCl = 0 (all of the 0.04 is used).
NH3 = 0.1-0.04 = 0.06 remaining, so what do we have?
We have a weak base (NH3) and its salt (NH4Cl). Do any of the others produce anything like that?
You can determine the concns if you wish from concn = moles/L and the final volume is 200 mL or 0.2 L.
Check my arithmetic. Sometimes I slip a decimal point when going between moles and molarity mostly because I do these in my head and work with millimoles and not moles.