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quick CHEMISTRY question....

abeal2

Senior member
Oct 7, 2004
208
0
0
Calculate the pH of ephedrine sulfate where the concentration of weak acid (ephedrine cation) is 10^-2 M. Kb for ephedrine base is 10^-5.
 

abeal2

Senior member
Oct 7, 2004
208
0
0
Originally posted by: James3shin
fine Ka by using the relationship Ka(Kb)=Kw then use henderson-hasselbach
so

Ka x Kb = Kw
Ka x 10^-5 = 10^-14
Ka = 10^-9
pKa = 9

pH = pKa + [A]/[AH]

i know pKa and [AH] (AH being 10^-2). how do i find [A]?
 

drinkmorejava

Diamond Member
Jun 24, 2004
3,567
7
81
lol reminds me that i have a chem test I should study for tomorrow,

I'd just solve for pOH then subtract it from 14 to get pH.
 

abeal2

Senior member
Oct 7, 2004
208
0
0
Originally posted by: drinkmorejava
lol reminds me that i have a chem test I should study for tomorrow,

I'd just solve for pOH then subtract it from 14 to get pH.
hehe, and how would i do that?
 

da loser

Platinum Member
Oct 9, 1999
2,037
0
0
Originally posted by: abeal2
Originally posted by: James3shin
fine Ka by using the relationship Ka(Kb)=Kw then use henderson-hasselbach
so

Ka x Kb = Kw
Ka x 10^-5 = 10^-14
Ka = 10^-9
pKa = 9

pH = pKa + [A]/[AH]

i know pKa and [AH] (AH being 10^-2). how do i find [A]?
i think that it's equilibrium, and since AH <-> A + H; A=H

i haven't done this in a long time, but i believe Ka or Kb is just a rate constant of the equilibrium, and since it's a weak acid the reaction does not go completely. so they give you AH, and that allows you to calculate how far the reaction goes using Kb
 

BD2003

Lifer
Oct 9, 1999
16,815
1
76
Originally posted by: da loser
Originally posted by: abeal2
Originally posted by: James3shin
fine Ka by using the relationship Ka(Kb)=Kw then use henderson-hasselbach
so

Ka x Kb = Kw
Ka x 10^-5 = 10^-14
Ka = 10^-9
pKa = 9

pH = pKa + [A]/[AH]

i know pKa and [AH] (AH being 10^-2). how do i find [A]?
i think that it's equilibrium, and since AH <-> A + H; A=H

i haven't done this in a long time, but i believe Ka or Kb is just a rate constant of the equilibrium, and since it's a weak acid the reaction does not go completely. so they give you AH, and that allows you to calculate how far the reaction goes using Kb
From what I can tell, it would have to be, otherwise there isnt enough info to solve the problem.
 

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