Question

A solution is made by mixing 0.150 M NH₄Cl and 0.100 M NH₃. Ka for NH₄+ is 5.61x10^-10 (please show math)

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

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

Here NH3 is strong base and NH4Cl is weak acid thus it is an example of basic buffer solution.

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

NH3 is a base.

NH3 + H2O = NH4+ + OH-

NH4Cl = NH4+ + Cl-

NH4+= NH3 + H+

Thus NH4+ is an acid.

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

No Cl- will not affect the pH of the solution only H+ and OH- concentration will affect the pH of the solution.

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

To calculate the pH of the mixture we use the Henderson-Haselbalch :
pH = pKa + log [NH3] / [NH4+]

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

pH = pKa + log [NH3] / [NH4+]

Given that

0.150 M= NH₄Cl and 0.100 M =NH₃. Ka for NH₄+ is 5.61x10^-10

pKa = -log Ka = - log 5.61x10^-10=9.25

pH = pKa + log [NH3] / [NH4+]

pH = 9.25 + log 0.100 / 0.150

pH = 9.25 + log 0.66

pH = 9.25 -0.180

pH = 9.07