Author Topic: Ionisation of water questions (Read 629 times) Tweet Share

jackinthepatch · « **on:** July 28, 2009, 07:34:31 pm »

Just need a bit of help with this:

Calculate K_w at 12.2^oC if pH is 7.3.

Thanks

TrueTears · « **Reply #1 on:** July 28, 2009, 07:35:44 pm »

Is it $2.5 \times 10^{-15}$ ?

jackinthepatch · « **Reply #2 on:** July 28, 2009, 07:39:37 pm »

Oh I dunno lol, we just have to use the results from our prac and work it out lol...how did you get that answer by the way?

TrueTears · « **Reply #3 on:** July 28, 2009, 07:45:56 pm »

Concentration = $10^{-pH}$

Water is always neutral (pure water) the $K_w$ value comes from $[OH^-][H_3O^+] = 10^{-14}$ at $25^o C$ .

Now say the temperature is at $25^o C$ we know that the pH of water is right in the middle of the pH scale for that temperature, ie, 7.

therefore $[H_3O] = 10^{-7}$ Since $[OH^-] = [H_3O] = 10^{-7}$ , so $K_w = 10^{-14}$ at $25^o C$ .

Using same principle.

$[H_3O] = 10^{-7.3}$

therefore $K_w = 10^{-7.3 \times 2} = 2.5 \times 10^{-15}$

jackinthepatch · « **Reply #4 on:** July 28, 2009, 08:01:04 pm »

Ah yeah thanks man

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Author Topic: Ionisation of water questions (Read 629 times) Tweet Share

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