A buffer is prepared by partially titrating 500.0 mL of 0.250 M ammonia, NH 3 (K

ID: 906072 • Letter: A

Question

A buffer is prepared by partially titrating 500.0 mL of 0.250 M ammonia, NH3 (Kb = 1.75 x 10-5) with 1.25 M HCl until a pH of 9.150 is reached.

(a) (0.2 pt) Write the balanced chemical equation for the titration reaction, omitting spectator ions.

(b) (0.1 pt) Can the NH4+ conjugate acid be produced without reacting any of the NH3 reactant?

Circle one: yes (explain how, below) no

(d) (0.1 pt) How many moles of NH3 are initially present?

(e) (0.2 pt) How many moles of the conjugate acid must be produced? How many moles of the strong acid must be added in order to do this?

(f) (0.2 pt) What volume of the strong acid will be added to the original solution? What is the final total volume?

(g) (0.2 pt) What are the concentrations of the conjugate acid and base in the final volume of the buffer?

a) NH3 + H+ ---> NH4+

with spectator ions the eqation is NH3 + HCl ---> NH4+ + Cl-

b) No

c) using the expression for pH of buffer as

pH = 14 - pKb - log [conjugate acid]/[base]

9.15 = 14 - 4.757 - log [NH4+]/[NH3]

or -0.093 = log [NH3]/[NH4+]

hence [NH3]/[NH4+] = 0.8072

d) Initial moles of ammonia = 500x0.25 x10-3 =0.125 moles

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.