If 75.0 mL of a 2.63 · 10-3 M NaOH is mixed with 125.0 mL of 1.80 · 10-3 M MgCl2, then calculate the reaction quotient and state if a precipitate will form? The Ksp of the expected precipitate is 1.2 · 10-11.

Yes, the precipitate of magnesium hydroxide will be formed.

Explanation:

Hello,

In this case, the undergoing chemical reaction is:

Whereas the precipitate is the magnesium hydroxide which is formed by:

Take into account that the solubility product is the inverse reaction. In such a way, the equilibrium is:

Thus, we know the concentration of OH⁻⁻ from the concentration of sodium hydroxide being the same (2.63x10⁻³M) since it is a strong base. Moreover, since magnesium chloride totally dissociates into magnesium and chloride ions the 1.80x10⁻³ M is also the concentration of magnesium ions.

Next, as the solutions mix, the final concentration of OH⁻⁻ is:

And the final concentration of Mg²⁺ is:

Finally, we compute the reaction quotient:

But the equilibrium constant:

Therefore, since K>Q, we infer that the precipitate of magnesium hydroxide (consider the procedure) will be formed.

Regards.