[chenewb]

Hi guys. Chemistry isn't a strong skill of mine. Could someone help me with this?

If I have 16 gallons of mostly-water, and I add to it 1/2 cup of ph 10.3 ash-water, and the resulting ph is 4.1, what was the ph before adding the ash-water?

part 2
Same question except instead of 16 gallons we start with 6.8 liters.

[Borek]

Not enough information. pH change will depend on the composition of "mostly water".

[chenewb]

Actually I think there is enough info there, he is just reacting to the "un-sciencey" word "mostly." Anyone else care to try? This is a simple a+2=3 question except weighted and on a logarithmic scale.

[Dan]

No, there is not enough information. The calculation depends on the pKa of the acid(s) present in the "mostly water", it is not possible to do the calculation without the identity of the acid(s) present. And you may have buffering effects of other components of "mostly water".

An illustration: You will get different numbers if "mostly water" is acetic acid solution compared with if "mostly water" is HCl solution.

It also depends on the size of the cup you are using.

It simply is not possible with the information provided.

[Arkcon]

If the "mostly water" is another liquid besides water, say alcohol, then that also screws up our ability to predict pH.

[DrCMS]

As chemistry is a skill of ours and in particular this area is speciality of Borek accept that the question you have asked can not be answered from the information provided.  In future learn that when you ask experts a question you might not get an answer you like but that does not make it wrong.