Question 16

16. For the strong acid-weak base titration of ammonia by hydrochloric acid, find the pH after 15.0 mL of 0.100 M HCl has been added to 25.0 mL of 0.100 M NH

₃ solution. (

K

_b = 1.8 x 10

^-5)

The reaction we will use is:

NH

₃ (aq) + H⁺ (aq)

NH₄⁺ (aq)

For the stoichiometric calculation:

	$NH$ ₃ (aq)	$H$ ⁺ (aq)	$NH$ ₄⁺ (aq)
INITIAL	0.0025mol	0.0015mol	0mol
CHANGE	-0.0015mol	-0.0015mol	+0.0015mol
FINAL	0.0010mol	0mol	0.0015mol

Converting to concentrations gives us $[ NH$ ₃ ] = 0.00250 M and $[ NH$ ₄⁺ ] is 0.0375 M.

The equation for this part is $NH$ ₃ (aq) + H₂O (l) NH₄⁺ (aq) + OH^- (aq)

The equilibrium constant expression is $K$ _b = {[ OH^- ](0.0375)} / {0.025} . Solving for $[ OH$ ^- ] we get 1.2 x $10$ ^-5. Which formula do we use to find the pH?