[nokia8650]

The problem is shown above.  I am able to do all of part (i), and the beginning of part (ii) (answer to part i is 10.37).

However, I do not understand the second part of question (ii) - "Suggest a method for the separation of magnesium hydroxide, but not calcium hydroxide, from the sample of sea-water. Explain your reasoning."

The answer is:  add NaOH - keep pH below 12.3, to prevent precipitation of Ca(OH) and to precipitate  Mg(OH)2.

Can someone please explain this answer - I do not understand it.

Thank you very much in advance.

[plankk]

As you said for the part (i) the correct answer is pH=10,37. What does it mean? What happens if the pH is below this value? And what happens if the pH is above this value?

[nokia8650]

If the pH is below this value, the equilibrium would shift over the right - producing hydroxide ions in solution.  If above it would shift over to the left, forming the precipitate.  So does this mean, that by keeping the pH above 10.37, but below 12.30, the equilibrium for dissolving Mg(OH)2 moves to the left - forming the precipitate, whilst that for dissolving Ca(OH)2 moves over to the right - keeping Ca(OH)2 in solution?

Thank you very much.

[Borek]

You may think about it this way - as long as pH doesn't reach 12.3 calcium hydroxide doesn't start to precipitate.