Question

1.A solution is made by mixing 0.150 M NH₄Cl and 0.100 M NH₃. K_a for NH₄⁺ is 5.61x10^-10

a.What type of solution you will have when the two compounds are mixed?

b.Identify the acid and the base (write the formula and the ID next to it)

c.Will Cl^- affect the pH of the solution? Why?

d.What is your prediction for the calculated pH compared to the pKa of the sample? Why?

e.What is the pH of the solution? Was your prediction correct?

Answer 1

1) we will get a buffer ( basic buffer solution ) when the above two are mixed NH3 act as base while NH4Cl act as a salt

2) [NH3] = base and NH4Cl = salt

3) using Henderson - Hasselbalch equation

pH = pKa + log [salt] / [acid]

pOH = pKb + log [salt] / [base]

yes the change in concentration of Cl- will effect pH of the solution . as we can see from above equation .

4) NH4OH <------> NH4+ + OH-

NH4Cl -------> NH4+ + Cl-

As Ka is directly proportional to the concentration of  NH4+ ions .

As  NH4OH is a weak acid it is partially dissociate and as NH4Cl is salt it completely dissociates .

By the common ion effect , the ionization of NH4OH is suppresed and thus there is excess of NH4+ ions and less concentration of OH .

As the pKa   = - log (5.61x10-10) =  9.2510 and the pH must be lower then this .

5) pOH = pKb + log [salt] / [base]

14- pH = 14- pKa + log [salt] / [base ]

pH = pKa - log  [salt] / [base ]

pH= - log (5.61x10-10) - log  0.150 / 0.100

= 9.2510 - log 1.5

=9.2510- 0.1760 = 9.075

yes the prediction was correct the pH is found to be lower then pKa .