Achieve #8

[305572238]

NH3 is a weak base ( Kb=1.8×10−5 ) and so the salt NH4Cl acts as a weak acid. What is the pH of a solution that is 0.081 M in NH4Cl at 25 °C?

Questions like this are difficult for me because I think that you have to write two equations to account for both NH3 and NH4Cl, but I have a hard time writing the equations. Also, in office hours a TA said that we can assume that the concentration of NH4Cl is equal to NH4+. Why is that?

[Natalie Keung 1D]

NH4Cl can be considered to be NH4+ since in water, the Cl reacts with the H in H2O and completely dissociates as it is a strong acid. This is why we completely disregard the Cl.
When writing the equation for this, you can put NH4+ and H20 on one side then try to determine if the weak base is likely to lose an H+ or gain an H+. Mostly, weak bases will result in the products OH- and the weak base minus one H+ and weak acids will result in the products H3O+ and the original acid with another H+.
For this problem, it asks for the pH but when you solve for X in the ICE table, you will get the concentration of OH, which you would then have to take the -log of that number and get pOH, then convert that to pH.
I hope this helps!

[905756606]

Hi! Since the question tells us that NH4Cl acts as a weak acid, we know that a solution of it will produce [H3O+] ions resulting in a pH<7. First, we can write out the dissociation of the salt when it dissolves: NH4Cl --> NH4+ + Cl- . Next, the strong conjugate base of the salt, NH4+, will react with water to form H3O+ : NH4+ + H2O ⇌ NH3 + H3O+. Finally, we use the ice tables to find the equilibrium concentrations and use Ka to solve for X. Once you have X you can find the pH as we have done before. Hope this helps!

[305607822]

Hi, thanks for all the responses, it helped me solve this problem! I am wondering, though, why do you use Ka to solve for X? Why couldn't you set Kb = [products]/[reactants] and find X that way?