Question

True or false question
The pH of 0.1mol/L acetic acid is 1.
So the amswer is False, the pH is greater than 1.
How is pH-log(0.1)=1 wrong?

I researched and found this solution but I don't know why I cant use pH=-log ([H+])

Ka=1.8×10^-5
1.8=x^2÷x
I get x=1.34 x 10^-3
pH=-log (1.34 x 10^-3)=2.87

Answer 1

I meant to type 1.8x10^-5=x^2÷x

Answer 2

I applaud your persistence as well as you resourcefulness. The answer you obtained for (H^+) of 1.34E-3 is correct for a 0.1M solution of acetic acid. You CAN(you really can) use pH = -log(H^+). What you aren't getting is that the (H^+) is 1.34E-3M and NOT (IT IS NOT, REPEAT, NOT) 0.1M The 0.1M is the concentration of the acetic acid and NOT the concentration of (H^+). That's because acetic acid is a WEAK acid and it is not ionized 100%. Solutions of 0.1M HCl are pH = 1 because (H^+) = 0.1. Weak acids don't ionize 100% so their (H^+) is less than 0.1 M for 0.1M solutions. From these data you can see that 0.1M acetic acid solutions only ionize 1.34%. Again, there is nothing wrong with pH = -log(H^+). The problem is that you are using (HAc) for (H^+) and for weak acids that isn't true. Hope this helps.