Question

Please show all your work!

Exp	Composition	Total Vol	pH
1	A few mL of 0.10M NH3 (unchanged)	n/a	11.13
2	2.5mL 0.1M NH3 & 7.5mL H2O	10 ml	10.82
3	2.5mL 0.1M NH4Cl & 7.5mL H2O	10 ml	6.01
4	2.5mL 0.1M NH3 & 2.5mL 0.1M NH4Cl & 5mL H2O	10 ml	9.26
5	Take 1mL Solution #4 and add 9mL of water	10 ml	9.26

Calculate the concentrations for solution 4 and solution 5

Sol	Conc. NH3 (M)	Conc.NH4+(M)	pH
2			10.82
4			9.26
5			9.26

Compare pH of solutions 2&4. What effect did adding NH₄⁺ have on the equilibrium/pH?

Compare the pH from dilution in solutions 1&2 to the pH change in solutions 4 & 5. Why is the pH change smaller for solutions 4&5 than in solutions 1&2? Think above the ratio of base and conjugate acid. You can also show this mathematically by using the Kb from part 1 with NH₃/NH₄⁺ conjugates to determine what the OH- concentration should be.

Answer 1

Soln	Conc. NH3	Conc. NH4+	pH
2	0.025 M	0 M	10.82
4	0.025 M	0.025 M	9.26
5	0.0025 M	0.0025 M	9.26

The pH of Solution 4 is less than Solution 2. Because, NH4+ is Acidic, and the Addition of NH4+ to NH3, makes a Buffer Solution.

In case of Solutions 1 & 2, the pH depends on the concentration of NH3. Whereas, in case of Solutions 4 & 5, the pH depends on the {[NH4+]/[NH3]} ratio, because both the Solutions act as a Buffer. Since, the Ratio remains same in Solutions 4 & 5, the pH also remains the Same, in both the Solutions.