[mufan]

Hi,
I have a solution that has a pH of 9 and want to take it to 8 using acetic acid.  How would I do that?  What about using HCL?

[enahs]

Add the appropriate amount.


It depends on the volume of your solution, not just the pH, and the concentration of your acid.

[mufan]

Let's say the volume of the solution is 1 liter and the concentration of the acetic acid is 5.0M.  What if the acid is HCL at 12.0M?

[Borek]

At such high pH this is simple stoichiomtery, just write neutralization reaction, calculate starting and final concentration of OH^- and calculate how much acid have to be added to remove enough OH^- for the change. It is as simple as that - unless pH 9 means there is buffer in the solution.

[mufan]

I'm too old and tired to try and figure all that out .  I know pH= -log[H+].  and I know mv=mv.  but I don't really know where to go from there.

The material that is pH of 9 is unknown.  I used a pH meter to test it.

[Borek]

If you have no idea what the solution composition is, you may try to add acetic acid slowly, controlling pH with pH meter. Without knwing what the solution contains it is not possible to say how much acid you will need.

[xps]

I have a similar one.

What mass (m) of NaOH (s) should I add to 1 L of [CH₃COOH]=0,20 mol/dm³ so that the ph of the total is 5?
Ka (CH₃COOH)= 1,8 x 10^(-5)

How's this done?

[Borek]

Please read forum rules.

This is buffer question - and as such requires Henderson-Hasselbalch equation. Use it to calculate how much acetate must be present, then it is simple stoichiometry.

[xps]

No, I know it can be done without Henderson-Hasselbalch equation. Just don't know how to do it.

[Borek]

Assume neutralization was quantitative - and calculate acid and salt concentration from simple stoichiometry. Then insert the values into HH equation.

See this buffer pH calculation example.