Acid/Base Concentration

[752779]

Hey guys I was watching Khan Academy, when they were calculating the pH of a strong acid: they were using pH= -log [H3O+] to calculate pH for the strong acid.

And here they used a different way to solve for pH in a weak acid. They were going to use the quadratic equation, but they have shorten it. However what I am confused is, these two questions seems to be similar, both asking to solve for pH of a solution providing concentration, but they used different method to solve for it. Is strong acid solve differently than weak acid?

---

Members don't see this ad.

 

[theonlytycrane]

For strong acids we can assume complete proton dissociation and just plug in the starting acid concentration for [H+] in the equation you described.

For weak acids, there is only partial proton dissociation so you use the ICE table method with a given Ka to estimate pH.

 

[752779]

For strong acids we can assume complete proton dissociation and just plug in the starting acid concentration for [H+] in the equation you described.

For weak acids, there is only partial proton dissociation so you use the ICE table method with a given Ka to estimate pH.

Gotcha.. thanks.. yeah they were using ICE method. to solve for the weak acid. thanks again..

 

[BerkReviewTeach]

The ICE method works, but is time consuming. Based on a few questions I've seen you post, it looks like you have access to the BR books. The BR method is significantly faster, so you really should use that for all pH and pI questions. It's demonstrated several times in the acids and bases section of the general chemistry book (Section 4 of the old books and Section 8 of the current books), but can also be found in the organic chemistry book as well (along with the pI questions).

 

[GJenks]

For solving for the weak acid ICE method is used