How Much Calcium Carbonate to Add for Optimal pH in a Fish Tank?

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The discussion centers on the calculation of calcium carbonate (CaCO3) required to adjust the pH of water in a fish tank from 5.0 to 6.5, specifically for breeding small fish that thrive in a pH range of 6.0 to 7.0. The calculations suggest that the mass of CaCO3 needed for 5 liters of water is complex due to the presence of buffers in the water, which complicates the straightforward neutralization process. The initial calculations indicate a need for 2.5x10^-2 moles of OH-, but the presence of other buffering agents renders the exact amount of CaCO3 required indeterminate without further information.

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marcelo
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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
 
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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,
 
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.

marcelo said:
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.
 
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