pH question...

[ice_blockie]

from STAV exam:

A student has 10 mL of a solution of KOH which has pH = 10. The solution is diluted with de-ionised water to make up a litre of solution. The pH of the diluted solution will be
A. 8
B. 9
C. 11
D. 12

The answer is A but why? This is what I did:







But then i did it another way:











Is it because you can't assume ?

[roly182]

Thats exactly right.. you cant assume that unless the solution is neutral/pure

[Mao]

the key is, at 25 degrees,

pH of 12 would be counter-intuitive - diluting a base makes it stronger???

[shinny]

For questions like this, just stick to using dilution factors. From 10mL to 1L (1000mL), you're diluting by a factor of 100 i.e. . Stick this number into the negative log formula for pH, and you'll realise that this represents a pH change of 2. Since you're diluting a base, obviously it's going to go down towards 7, so it'll go from 10 to 8.

[lanvins]

what would you do if it were made more concentrated?

[onlyfknhuman]

For questions like this, just stick to using dilution factors. From 10mL to 1L (1000mL), you're diluting by a factor of 100 i.e. . Stick this number into the negative log formula for pH, and you'll realise that this represents a pH change of 2. Since you're diluting a base, obviously it's going to go down towards 7, so it'll go from 10 to 8.

nice way of thinking (Y)

[lanvins]

v