Changing the pH of a Swimming Pool.

[Gaunt101]

Homework Statement

A swimming pool with 95000L of water with a pH of 8.9 needs to be lowered to a more suitable pH using an acidic substance. An ideal pH would be within the range of 7.2-7.6, so let's just assume that the pH we're hoping to achieve is 7.4. My brother chose trichlorocyanuric acid (TCA) as his substance to lower the pH, although I"m not entirely sure of the calculations involved since I've been unable to find a Ka value for TCA.

Homework Equations

pH = -pKa + log([A-]/[HA-])
c = n/v
n = m/M
Ka = [A-][H+]/[HA-]

The Attempt at a Solution

I assumed you would just use the first equation? I'm not entirely sure to be honest, the only way I can imagine doing this question is by assuming the Ka value of cyanuric acid and as a result you can the rearrange the first equation and solve for the concentrations of each species. Although I'm almost certain that this approach is incorrect. I'm terribly sorry I'm not particularly sure where to start.

 

[Borek]

Substance you add is only part of the problem. Not knowing what makes your pool water pH to be 8.9 you can't solve the question.

 

[tiny-tim]

we goldfish consider changing the ph of swimming water to be a form of terrorism

 

[Gaunt101]

Thanks for responding Borek and Tiny Tim!

I believe my brother was allowed to choose and he has chosen sodium carbonate Na₂CO₃. How should I start considering this problem? Would it just be trichlorocyanuric acid + Na2CO3 <--> ? and then find a Ka value of cyanuric acid?

Sorry for not being able to have a more directed question :(.

 

[Borek]

You don't know enough to solve the question, period. The only way to determine amount of whatever substance that needs to be added is an experiment.

 

[Gaunt101]

Theoretically could you solve the question if you knew the measurements of how much sodium bicarbonate had fallen in right? I'm guessing since this is an assignment I can probably just model a realistic scenario and apply suitable values? Is there a way to solve the question if you knew the measurements? If not... then how what would you need to know, to solve the question, if I can just understand the process that'd be awesome ^_^.

Thank you so much for your time!



Gaunt.

 

[Borek]

You need to know alkalinity of the solution, or its buffer capacity. These are ways of expressing how much acid (or base) needs to be added to change pH of the solution. Assuming carbonate is the only source of pH you in theory estimate amount of acid that has to be added, but accuracy would be so low calculations are a waste of time.