The compound para–aminobenzoic acid (PABA) is a powerful sun–screening agent whose salts were once used widely in sun tanning and screening lotions. The parent acid, which we may symbolize as H–Paba, is a weak acid with a pKa of 4.92 (at 25 °C). What are the [H+] and pH of a 0.030 M solution of this acid? Please help!!

Question

aaronq · Answer

So write an equation for the dissociation of the acid, then use it to write an equilibrium expression.

$\sf pK_A=-log(K_A)$

anonymous · Answer

i thought that you would find H+ by multiplying 4.92 and 0.030 and then for the ph you would solve for x?

aaronq · Answer

it's not too far off. multiply the Ka by 0.03 then take the squared root, then you have [H+].

Next take the negative logarithm of [H^+] then you have the pH

anonymous · Answer

i got -.021 for H+? is that right?

aaronq · Answer

no it can't be negative

anonymous · Answer

okay so would it be .021 then?

anonymous · Answer

i did the -log of 4.92 and then i multiplied it by the0.030

aaronq · Answer

the first step is wrong, the formula is $\sf pK_A=-log(K_A)$, so then $\sf K_A=10^{-pK_A}$

anonymous · Answer

okay i got 3.61?

aaronq · Answer

what is that?

anonymous · Answer

for H?

aaronq · Answer

Does that make sense to you? 

the initial concentration of the acid is 0.030 M

anonymous · Answer

yes

aaronq · Answer

no it can't be higher than the concentration of the acid

aaronq · Answer

did you do what i said previously?

"multiply the Ka by 0.03 then take the squared root, then you have [H+]"

anonymous · Answer

yeah you said ka was equal to 10^-4.92? and then i multiplied it by 0.030? and then i took the square root of it and got 6.00x10^-4

aaronq · Answer

yeah, that answer is right

aaronq · Answer

it's not what you wrote before though

anonymous · Answer

yeah i forgot to take the square root

anonymous · Answer

and then for the ph i get 3.22?

aaronq · Answer

yep that's right

anonymous · Answer

yay! thanks!!

aaronq · Answer

no problem !