Calculate pH/pKa: pH 4.0, HA Aqueous Solution

[Huskies213]

First is this correct?

The pH of a 150 mL aqueous solution of 2.13 x10^-3 M HCl is
pH = -Log (2.13 x10^-3)
pH = 2.672

and how would you solve this one?

A .1M aqueous solution of an acid HA has a pH of 4.0 what is the pKa of HA?

 

[PPonte]

First is, indeed, correct.

and how would you solve this one?

A .1M aqueous solution of an acid HA has a pH of 4.0 what is the pKa of HA?

I cannot help until you express some of your thoughts. Sorry, I must respect the rules.

Suggestion: What information gives you the question when she says the pH is 4 to help you find Ka and, then, pKa?

Check: http://en.wikipedia.org/wiki/PKa

 

[Borek]

2.67, as you used more significant digits in your answer than you had in the input data.

As for the second question - what formula do you use for pH calculation? Can it be rearranged/solved for pKa?

Check these pH calculation lectures.