2 chem questions

1. can we prepare soluble salts by precipitation? (get the filtrate after separating the precipitate by filtration)

It can be done like this :

BaCl₂(aq) + MgSO₄(aq) --> MgCl₂(aq) + BaSO₄(s)

After filtration, the filtrate basically contain MgCl₂. Evaporate off the solvent (water). Solid MgCl₂ can be obtained.

It is a possible way, but may be not a very good way. We say BaSO₄ is insoluble and can be removed by filtration, but actually it dissolves a little bit. The Ksp of
BaSO₄ is 1.1X10⁻¹⁰, a solubility of 0.0023 g / dm³. What i mean is this way to obtain a soluble salt would contain impurities. So. this is possible but may be not a very good method.

Alternatively, MgCl₂ can be obtained by reacting the metal Mg with HCl. This will contain less impurity.

2. when sodium chloride is added into an alkaline solution, what will be the change in pH value?
Is the pH value of a solution depends solely on concentration of H+ ions (but not OH- ions)?

If the sodium chloride added is in solid form. It doesn't affect the volume much, the pH remains unchanged. ( no dilution effect).

pH is defined solely by hydrogen ion concentration.

It seems OH⁻ does not affect pH. But actually this is not the case.

[H⁺] = hydrogen ion concentration
[OH⁻] = hydroxide ion concentration

Actually there is a relation called ionic product of water.
It says that [H⁺][OH⁻] = 1X10⁻¹⁴

Hydrogen ion concentration change would change the hydroxide concentration at the same time. they are not independent things.

So. pH is defined by hydrogen ion concentration. But at the same time, it also fixed the hydroxide concentration by the relation. [H⁺][OH⁻] = 1X10⁻¹⁴

2. pH = - log [H+]; pOH = - log [OH-]; pH + pOH = 14 at specified condition