Achieve Week 1 #1

[405513470]

If the Ka of a monoprotic weak acid is 7.6×10−6, what is the pH of a 0.21 M solution of this acid?
How are we supposed to go about finding the concentration of H+ if the acid and its reaction is not given to us?

[Nancy Romo 1E]

You could set up a general chemical reaction of an acid, which is by definition a chemical substance that has an H proton to donate:

HA (aq) + H2O (l) = H3O+ (aq) + A- (aq)

And from there, you could go about solving the ICE table since you are given the initial concentration of the [HA] acid. It may be helpful to refer to the textbook section 6D.1 for weak acids, hope this helps!

[Isabelle Rieke-Wey 2G]

Hi!
to solve this problem, I would start by setting up the balanced chemical equation for an acid: HA +H2O=H3O+ + A-
Then, you can use an ICE table using the initial concentration of HA (0.21M) in order to solve for the concentration of H+.

[005502505]

Hi! Like the other responders, I made an ICE table and solved for x using the Ka expression to find H concentration. Hope this helps!

[Shania Garrison Discussion 3E]

What I did was I just chose a monoprotic weak acid I knew (CH3COOH) and wrote the reaction for that. Then, I basically just used that to solve each of those problems. When there was a weak base, I just used a weak base I new like NH3.

[Caroline 2A]

Hi!

First, start by assembling an ICE table. Because the value of M is more than 1000 times greater than the Ka, we are able to assume that the concentration change x is sufficiently small compared to the initial concentration. We can check this by dividing the value of X by the initial concentration and multiplying that value by 100%. If x is less than 5% of the concentration, then our assumption is valid (and it is valid in this case).

From there, we can find the pH by using the equation pH= -log[H+] where x is equal to H+. This should give you your final answer.

Hope this helps :)

[Hannah Jin 1J]

Setting up an ice table, using x as the change in concentration from reactants to products, setting up an equation using x to equal the Ka value, and then solving for x should get you the value you need to find the pH.