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# Kw help Watch

1. ...........................lp
2. I prefer to use pH + pOH = 14 (which comes from the Kw formula) for these type of calculations

edit: where pOH = - log [OH-]
3. ...............................
4. (Original post by sam2109)
ok was tht a hint becasue u have jus stated what i stated above?
to calculate OH we divied kw value by 10^-13.5?
Did I, I don't think so?

if pH = 9 then pOH = 5

as so [OH-] = 10-5 mol dm-3
5. .................
6. (Original post by sam2109)
is the answer 0.316?
no.
7. (Original post by Daniel-Ballingall)
no.
whats the principle to work it out,
jus make ur own example
8. (Original post by sam2109)
whats the principle to work it out,
jus make ur own example
pH = concentration of H ions.

pH of 9 = 10^-9 hydrogen ions

has to equal 10^-14

therefore the conc. of OH ions is 10^-5.

Where did you get your answer from?

ps. I repped your post.
9. ..........................
10. ...........
11. (Original post by sam2109)
alright so the concentration of NAOH is 1*10^-5,
if that was the case why did they give us the value of kw, if the pH value was 13.5 the concentration of NAOH would be 0.316?
I don't undertsant what you are saying.
Kw is always 10^-14, they don't give you that, you should just know it.

"if the pH value was 13.5 the concentration of NAOH would be 0.316?"
... yes.

Quoting me twice doesn't make me reply quicker.
12. ................
13. (Original post by sam2109)
thank you i changed my original question and kept referrring to its answer on the answers i gave.

the question later says that the pH meter has an error of + or - 1 pH unit, what is the maxiumum concentration of NaOH
what do i do here
thanks again for ur help
plug in the value for the highest value of pH possible (including error) and work out the [OH-] for this pH

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