Calculate pH of 2.0e-8M NaOH in Water

[gunnar]

there are 2.0e-8 M NaOH in water. I have to calculate the pH of the solution considering the OH from the water. The answear to this is pH=7.04 I know this is calculated with the quadratic equation. what I don't get is how to set this up. I alway get a wrong answear. Can somone help me with this?

 

[Borek]

there are 2.0e-8 M NaOH in water. I have to calculate the pH of the solution considering the OH from the water. The answer to this is pH=7.04 I know this is calculated with the quadratic equation. what I don't get is how to set this up. I alway get a wrong answear. Can somone help me with this?

You have two equations:

Kw = [OH^-][H⁺]

and charge balance:

[OH^-] = [H⁺] + [Na⁺]

This is enough to solve the problem, but in case you don't know what to do next:

You have to get rid of [OH^-], so rearrange first equation

[OH^-] = Kw / [H⁺]

and use [OH^-] in the second equation:

Kw/[H⁺] = [H⁺] + [Na⁺]

That's your quadratic equation.

Try BATE for such (and much more complicated) pH and acid - base equilibrium calculations.
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BATE - pH calculations, titration curves, hydrolisis

 

[gunnar]

Worked out fine. Thanks a lot