Salt Solution pH

[arisawaters2D]

There was an example from a lecture that I'm still a little shaky on. Can someone walk me through how to solve this problem:

Given Kb for NH3 is 1.8*10-5, what is the pH of 0.15 M of NH4Cl- ?

[Sandy Lin 1L]

To find the pH of NH4Cl, you would write out the equation of this salt in water. Since Cl- is just a spectator ion, you can leave it out so the equation you will get should be NH4+(aq) + H2O(l) produces H3O+ (aq) + NH3(aq). Now you can set up an ICE chart with 0.15M as the initial concentration of NH4+ and use this to write out the expression for Ka. Since they only give you the Kb value, you have to convert it using Kb x Ka = 10^-14 because NH4+ is acting as an acid. With this, you should be able to calculate the H3O+ concentration, so take the -log to find pH.

[Lily Kiamanesh 2G]

For these types of problems, it's important to first determine which species is acting, the acid or the base. In this case, it's the acid. Once that is determined, you use an ICE table and either the Ka or the Kb (in this case the Ka) and solve normally.

[alette1a]

Sandy did an excellent job explaining the problem. I just want to highlight a couple of pointers when solving these kinds of problems:
- Make sure you use Ka for acids and Kb for bases.
- Use Ka*Kb=Kw to figure out Ka or Kb.
- Ka and Kb that correspond to each other are related through conjugates; so a conjugate base's Kb is found by doing Kw/Ka of the conjugate acid.
- You can find the spectator ion by asking yourself: when it's in the formula for an acid or a base, is this acid or base strong? If the answer is yes, then the ion is essentially neutral in solution and thus is a spectator.

[Ellison Gonzales 1H]

When the problem states the reaction is happening @25 degrees, is this an indicator it is a neutralization? I noticed a pattern in some problems but I’m not sure if it is true or accurate

[Kaihan_Danesh_2J]

First, you would divide the Kb value by 1*10^-14 to convert it to a Ka value because, now, you are dealing with the conjugate acid of nh3, nh4+. Then, you would set up the Ka equation for the reaction between nh4 and water to hydronium and nh3. Finally, if you use an ice table, you'll see that hydronium and nh3 increase by "x" and nh4 decreases by "x", but nh4 has an initial concentration of 0.15 M. So, the equation you should get is x^2/(0.15-x) = Ka. Then, you solve for x and since the concentration of hydronium is "x", then taking the negative log of x will give you pH.

[JasmineCap1A]

When the problem states the reaction is happening @25 degrees, is this an indicator it is a neutralization? I noticed a pattern in some problems but I’m not sure if it is true or accurate

25 degrees is just a standard set for your table if K changes with with temperature.