Question

Calculate the amount of calcium dissolved if the pH becomes 7.50. Is it better to have a high or low pH, if one is interested in not dissolving their teeth?

Answer 1

You should have posted the whole question. I think we have only posted a part of question. Here you did not mention which Ca compound you are talking about. I search the internet I found that the question has a mention of following Ca compound-

Ca₁₀(PO4)₆(OH)₂. (Please change your solution if you have a different calcium compound, the method will be the same)

The easiest way of solving this part of question is to write the dissociation equation of the the above compound

Ca₁₀(PO4)₂₍OH) = 10Ca⁺² + 6PO₄^-3 + 2OH^-

Here you see that the complete dissociation or dissolution of 1 mol of above compound will yield 10 mols of Ca⁺² along with 6 mols of PO₄^-3 and 2 mol of OH-. But since the above compound is only partially dissociated therefore, OH- concentration can be calculated based on the pH of solution.

Now, pH is given as 7.5, which means pOH = 14 - 7.5 = 6.5, therefore, [OH-] = 10^-6.5 = 3.16 x 10^-7 M

Now, dissolved concentration of Ca⁺² will be 5*[OH-] = 5*3.16 x 10^-7 = 1.58 x 10^-6 M

We want to have high pH to protect our teeth from dissolving because low pH will increase the dissolution of Ca₁₀(PO4)₆(OH)₂.