Homework Help: College Chemistry

Posted by Kyle on Tuesday, March 23, 2010 at 6:22pm.

Amphetamine (C9H13N) is a weak base with a pKb of 4.2.

Calculate the pH of a solution containing an amphetamine concentration of 215 mg/L.

I calculated the molarity as 1.58*10^-3 and the Ka as 1.58*10^-10. I did ICE and i got the pH as 6.3, but it was wrong. Do you know what I did wrong?

College Chemistry - DrBob222, Tuesday, March 23, 2010 at 7:16pm
I think you treated amphetamine as a salt. Its a base. pKa of 4.2 makes Kb = 6.3 x 10^-5. If we represent amphetamine as RN, then
RN + HOH ==> RNH^+ + OH^-

Kb = (RNH^+)(OH^-)/(RN)
6.3 x 10^-5 = (x)(x)/1/59 x 10^-3
solve for x which is (OH^-), convert to pOH, then to pH. I get something like 10.5 but that's a quickie. Check it out.

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