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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?

  • Chemistry -

    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.

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