Final answer:
When mixed, the pair of aqueous solutions MgCl2 + KOH will form a precipitate.
Step-by-step explanation:
When two aqueous solutions are mixed, a precipitate may form if the resulting compound is insoluble in water. In this case, we need to determine which pairs of solutions will form a precipitate. Let's analyze each option:
a) HI + KOH: When hydroiodic acid (HI) and potassium hydroxide (KOH) react, they form potassium iodide (KI) and water (H2O), which are both soluble in water. Therefore, no precipitate will form.
b) K2S + HBr: When potassium sulfide (K2S) and hydrobromic acid (HBr) react, they form potassium bromide (KBr) and hydrogen sulfide (H2S). Hydrogen sulfide is a weak acid and will partially ionize in water, producing H+ ions, while KBr is a soluble salt. Since no insoluble compound is formed, no precipitate will form in this reaction either.
c) K2CO3 + HNO3: When potassium carbonate (K2CO3) and nitric acid (HNO3) react, they form potassium nitrate (KNO3) and carbonic acid (H2CO3). Carbonic acid is a weak acid and will partially ionize, so H2CO3 will decompose into H2O and CO2. Both KNO3 and CO2 are soluble in water, so no precipitate will form in this reaction.
d) MgCl2 + KOH: When magnesium chloride (MgCl2) and potassium hydroxide (KOH) react, they form magnesium hydroxide (Mg(OH)2) and potassium chloride (KCl). Magnesium hydroxide is only slightly soluble in water, with a low solubility product, so it will form a precipitate. Therefore, the pair MgCl2 + KOH will form a precipitate when mixed.