A solution is prepared mixing 5.0 mL of .10 M NH3, 2.00 mL .125 M HCL, and 3.00

Question

A solution is prepared mixing 5.0 mL of .10 M NH3, 2.00 mL .125 M HCL, and 3.00 mL H20.

A. Calculate the concentration of NH3 and NH4+ ions after the mixing, considering a quantitative reaction btween NH3 and H+, but before the solution equilibriates.

B. Calculate the pH of this solution, using the Henderson-Hasselbalch equation, from the known concentrations of the two major species present after the reaction above.

C. According to the Henderson-Hasselbalch equation, how will the pH of this buffer change if the buffer solution is diluted 2-fold with water?

D. You should recall that the Henderson-Hasselbalch equation breaks down eventually. So not using the Henderson-Hasselbalch equation, calculate the new pH if the original buffer solution is diluted 5-fold with water.

E. What is the pH if it is diluted infinitely with water.

Explanation / Answer

NH3+ HCl ---> NH4+ + Cl-

A)concentration of NH4+ =(2*0.125)/(2+5+3)

=0.025 M

concnetration of NH3=(5*0.1-2*0.125)/(2+5+3)

=0.025 M

B)pOH=pKb+log(salt/base)

=4.76+log(0.25/0.25)

=4.76

so pH=14-pOH

=14-4.76

=9.24

c)if the buffer is diluted with 2 times more water, the pH of the solution would not change since the number of moles of NH3 and NH4+ ions remain the same.

d)if diluted with 5 times the water,

pH=8.3

E)on infinite dilution,

pH=7

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