Question

The labels have fallen off two bottles, one containing \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\) and the other containing \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\). You have a bottle of dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\). How could you use it to test a portion of each solution to identify which solution is which? [Section 4.2]

Short answer

Add dilute sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) to each unknown solution. If the resulting solution is clear, the initial solution is magnesium nitrate (\(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\)). If a precipitate forms, the initial solution is lead nitrate (\(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\)).

Step-by-step solution

Write down the chemical reactions with sulfuric acid

We will add dilute sulfuric acid to both solutions: 1) Magnesium nitrate reacts with sulfuric acid: \(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2} + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{MgSO}_{4} + 2\mathrm{HNO}_{3}\) 2) Lead nitrate reacts with sulfuric acid: \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{PbSO}_{4↓} + 2\mathrm{HNO}_{3}\)

Analyze the results of the reactions

Now, let's analyze the products of the reactions: 1) Magnesium sulfate (\(\mathrm{MgSO}_{4}\)) is a soluble salt and will dissolve in water, forming a clear solution. Nitric acid (\(2\mathrm{HNO}_{3}\)) is also a soluble compound. 2) Lead sulfate (\(\mathrm{PbSO}_{4}\)) is an insoluble salt and will form a precipitate when mixed with sulfuric acid. Nitric acid (\(2\mathrm{HNO}_{3}\)) is also a soluble compound.

Perform the test and identify the solutions

In order to identify which solution is which, we will add the dilute sulfuric acid to each unknown solution: 1) If the resulting solution is clear after adding sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)), then the initial solution is magnesium nitrate (\(\mathrm{Mg}\left(\mathrm{NO}_{3}\right)_{2}\)). 2) If the resulting solution forms a precipitate after adding sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)), then the initial solution is lead nitrate (\(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\)).