PH fish tank water problem

[marcelo]

Considering a fish breeder decided to breed small fishes which needs a pH between 6,0 to 7,0 to stay alive. He needs to adjust the water's pH that is 5,0 to a value of 6.5, having available only calcium carbonate. The mass in mg added to 5L of water is about:
A)2,5

B)5,5

C)6,5

D)7,5

E)9,5

 

[DaveC426913]

What do YOU think?

 

[marcelo]

I think if I have 10-5 of pH value we need 10-9 of pOH value to make Kw=-14
the reaction of water with CaCo3 is 2 per 1
(H+) + (OH-) + CO3(2-) + Ca(2+)----> HCO3(-) + Ca2(+) OH(-)
(H+) + (OH-) +HCO3(-) + (Ca(2+) + OH(-)-----> H2CO3 + Ca(OH)2
since I need only 10^-2 concentration of OH- and we have only 5L of water:

5.10(-2) mols of OH-. But 1 mol of CaCO3 gives 2 mols OH, we need only 2.5x10(-2) but this answer is wrong,

 

[Borek]

As asked this question has no answer, as we don't know what is responsible for the initial pH - typically water contains some buffers and the amount of base/acid required to change the pH depends of their concentration. Then, adding carbonates creates another buffer set, making the calculation a bit more convoluted than just the neutralization would do.

I think if I have 10-5 of pH value we need 10-9 of pOH value to make Kw=-14

This is way too cryptic for me to guess what you mean. Or, if I guess right, you are mixing pH with [H⁺] and so on as if they were the same thing - they are not.