## anonymous one year ago 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!!

1. aaronq

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

2. anonymous

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

3. aaronq

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

4. anonymous

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

5. aaronq

no it can't be negative

6. anonymous

okay so would it be .021 then?

7. anonymous

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

8. aaronq

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

9. anonymous

okay i got 3.61?

10. aaronq

what is that?

11. anonymous

for H?

12. aaronq

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

13. anonymous

yes

14. aaronq

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

15. aaronq

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

16. anonymous

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

17. aaronq

18. aaronq

it's not what you wrote before though

19. anonymous

yeah i forgot to take the square root

20. anonymous

and then for the ph i get 3.22?

21. aaronq

yep that's right

22. anonymous

yay! thanks!!

23. aaronq

no problem !