Anyone seen any tricky/weird pH problems

[axp107]

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1) 1 type is when they give you something like this:

pKb of NH2- is x
pKb of NH3 is y

then, they ask what is the pKa of NH3?

its NOT 14 -y, its 14-x

2) Has anyone seen a "Find the pH of a weak acid in water problem?" ESPECIALLY on the AAMCs

For example:

Find the pH of 0.01 M HCN given its ka?

Sometimes.. you're going through the PS section so fast, that you might not even think.. hey wait a minute, HCN is not a strong acid.. it doesn't disassociate completely. You put 2 down as your answer.. BAM! Wrong! 😀

You'd have to do:

ka = [x][x]/ (0.01-x)

Has anyone ever had to find the pH of a weak acid on the MCAT or AAMCs?

Feel free to add more acid/base problems that you find tricky or that people miss easily

 

[DocWalken]

I haven't started studying yet, but those don't seem too bad. I would be more concerned if there were asking you to calculate pH of multiprotic acids during a titration or something like that. Does anyone know if these kinds of problems are on the test?

 

[quiksilver87]

1) 1 type is when they give you something like this:

pKb of NH2- is x
pKb of NH3 is y

then, they ask what is the pKa of NH3?

its NOT 14 -y, its 14-x

2) Has anyone seen a "Find the pH of a weak acid in water problem?" ESPECIALLY on the AAMCs

For example:

Find the pH of 0.01 M HCN given its ka?

Sometimes.. you're going through the PS section so fast, that you might not even think.. hey wait a minute, HCN is not a strong acid.. it doesn't disassociate completely. You put 2 down as your answer.. BAM! Wrong! 😀

You'd have to do:

ka = [x][x]/ (0.01-x)

Has anyone ever had to find the pH of a weak acid on the MCAT or AAMCs?

Feel free to add more acid/base problems that you find tricky or that people miss easily

hey axp,
About the second one...yeah I'm pretty much very used to those questions by now! Usually the ka of weak acids on the MCAT are very low, so you can drop the -x factor in the denominator...This makes the math much easier!

The first question is a bit tricky...You must make sure you know what Kb is and what Ka is...That question actually got me at first...

http://library.thinkquest.org/C006669/data/Chem/equilibrium/kb.html

 

[_T_]

I like this thread mostly cause I'm horrible at acid/base stuff lol..any suggestions on where to look for improvement? I've read both Kaplan and EK but I'm still missing questions

 

[axp107]

I like this thread mostly cause I'm horrible at acid/base stuff lol..any suggestions on where to look for improvement? I've read both Kaplan and EK but I'm still missing questions

Look at the acid/base EK 30 minute end of chapter exam.. take it.. its hard.. but try and understand every question after taking it.. that probably helped the most 👍