Which of the following mixtures will result in the formation of a buffer solution if all solutions are 1.0 M before mixing?

i) 100. mL of NH3(aq) and 100. mL of HCl(aq).
ii) 100. mL of NH3(aq) and 50. mL of HCl(aq).
iii) 100. mL of NH3(aq) and 50. mL NaOH(aq).
iv) 100. mL of NH4Cl(aq) and 100. mL of NH3(aq).
v) 100. mL of NH4Cl(aq) and 50. mL of NaOH(aq).

a. ) i and ii
b. ) i and iv
c. ) ii and v
d. ) ii, iii, and v
e. ) ii, iv, and v

I strongly think it's (ii) and (v), but I'm unsure of (iv)

3 answers

For a basic buffer you want a weak base (NH3) and a salt of the weak base (NH4Cl) since the acid being mixed is always HCl.
Just reason through to see if you have a weak base and its salt.
NH3 + HCl ==> HCl
i. 100 mmoles (mL x M) NH3 and 100 mmoles HCl. Exactly neutral, no excess of either; therefore, no buffer.

ii. 100 mmoles NH3 + 50 mmoles HCl gives 50 mmoles NH4Cl and uses 1/2 leaving 50 mmoles NH3. Both NH3 and NH4Cl present; therefore, buffer.

iii. 100 mmoles NH3 and 50 mmoles NaOH, no salt formation; therefore, no buffer.

iv. 100 mmoles NH4Cl + 100 mmoles NH3. Both base and salt present; therefore, buffer.

v. 100 mL NH4Cl + 50 mmoles NaOH.
NH4Cl + NaOH ==> NaCl + H2O + NH3
All NaOH used.
produces 50 mmoles NH3 and leaves 50 mmoles NH4Cl; therefore, NH4Cl and NH3 present in final mixture and it is buffered.
OK?

Thank you

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