Achieve week 2 #9 and #10

[Aren_Kasparian_1G]

How can we determine whether a species will be charged or not in a solution at a certain pH?

[Michael Crannell 1H]

I also had this question- What I found through doing problems was that an Acid <pH 7 is usually charged and vice versa for a base? Is this correct or do I have it backwards maybe?

[Christina Gigoux 1D]

You determine if it will be charged by comparing the pH of the acid/base with the solution. If a solution is more acidic than an acid, the acid will not be charged, and similarly, if a solution is more basic than the base, then the base will not be charged. Conversely, if a solution is less acidic than an acid, the acid will be charged, and if a solution is less basic than a base, the base will be charged.

[claire smith]

When this problem talks about a species being charged, it is referring to whether other not the substance will be in its acid/base or conjugate base/acid form. Start by figuring out which of those two would be the charged molecule and which would be neutral. From there, you can use a comparison of pK and pH to figure out if the protonated or deprotonated substance will be more likely to form.

[Yuhan Sun]

For weak acid:
HA(aq)+H2O(l)↽−−⇀A−(aq)+H3O+(aq)
pH equal pKa & pH less than pKa, it would be neutral. Because HA will become more predominant in solution.
pH greater than pKa, it would be charged. Because A- will become more predominant in solution.

For weak base:
B(aq)+H2O(l)↽−−⇀BH+(aq)+OH−(aq)
pH equal pKa & pH greater than pKa, it would be neutral. Because B will become more predominant in solution.
pH less than pKa, it would be charged. Because BH+ will become more predominant in solution.

[Molly McAndrew 1 1H]

Both of these problems use a comparison of pH and K_a. Will we ever be given K_b and/or pOH, and if so, do all of those assumptions just change?

[madison_bang_1L]

I find that thinking about these questions conceptually helps eliminate the confusion with the whole mess of numbers. If pH is less than pKa, for example, we know that it's more acidic and thus contains more [H3O+]. To minimize this excess of products, the solution will shift towards the reactants. When favoring the reactants, we know the solution will be neutral because the original acid is [HA] and uncharged. If pH is greater than pKa, then the solution is more basic and therefore favors the products in order to increase the concentration of [H3O+].

Hope this helps!

[Matthew Li 1B]

set the Pka in the middle and if the depending on the pH and whether there is a weak base or acid. If the pH is greater than Pka and the solution is a base, then it will be neutral

[Sidney Shah 3H]

Adding on to what other people said, the answer also included this helpful diagram that helps to show the relationship between pKa and pH