pH have to be set lower or higher than 7.4? At 50 celcius, the pKw of water is 13.262. If you wanted to adjust the pH of the 50 celcius solution so when the solution cooled to room temperture (25 celcius) it would have a pH of 7.4. Would you have to set the pH to a value greater or less than 7.4? Explain.

Expert Answers

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The expression for the kw of water at any temperature is:

`Kw = [H^+] [OH^-]`

if pKw = 13.162

Kw =  `5.7406x10^(-14)`

`Kw = [H^+] [OH^-]`

`5.706x10^(-14) = [H^+] [OH^-]`  

`if x = [H^+] = [OH^-]`

` ` `5.706x10^(-14) = x^2`

`2.3388x10^(-7) = x`

`pH (at 50^o) = -log[H^+]`

`pH (at 50^o) = -log (2.3388x10^(-7))`

`pH (at 50^o) = 6.631`


At 25 degrees Celsius, the pH of water is 7.0 while 

at 50 degrees Celsius, the pH of water is 6.631

If we want to make the pH higher at 25 degrees Celsius, we should make the pH higher as well at 50 degrees Celsius. Our limit then is 7.4; we should only increase the lower than 7.4.

Because if you increase the pH of water at 50 degrees to 7.4, the resulting pH of the water at 25 degrees Celsius would be higher than 7.4.


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