Question

An aqueous solution of an unknown solute is tested with litmus paper and found to be acidic. The solution is weakly conducting compared with a solution of \(\mathrm{NaCl}\) of the same concentration.Which of the following substances could the unknown be: \(\mathrm{KOH}, \mathrm{NH}_{3}, \mathrm{HNO}_{3}, \mathrm{KClO}_{2}, \mathrm{H}_{3} \mathrm{PO}_{3}, \mathrm{CH}_{3} \mathrm{COCH}_{3}\) (acetone)?

Short answer

The unknown solution could be \(\mathrm{H}_{3}\mathrm{PO}_{3}\) since it is acidic and has weaker conductivity compared to a solution of \(\mathrm{NaCl}\) at the same concentration.

Step-by-step solution

Identify acidic substances

Among the substances listed, we need to find those that have acidic properties. Recall that acidic substances increase the concentration of hydrogen ions (\(\mathrm{H}^{+}\)) in the solution. We can identify these substances based on their chemical formulas, specifically those that can donate an \(\mathrm{H}^{+}\) ion. Below, we list the substances and their acidic properties: - \(\mathrm{KOH}\): Not acidic (basic) - \(\mathrm{NH}_{3}\): Not acidic (basic) - \(\mathrm{HNO}_{3}\): Acidic - \(\mathrm{KClO}_{2}\): Not acidic (neutral salt) - \(\mathrm{H}_{3}\mathrm{PO}_{3}\): Acidic - \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (acetone): Not acidic From this analysis, we find that \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{3}\mathrm{PO}_{3}\) are acidic substances and could be the unknown acid in the solution.

Consider conductivity properties

Now that we have identified the acidic substances, we will need to consider their conductivity properties. Remember that the unknown solution is weakly conducting compared to a solution of \(\mathrm{NaCl}\). Thus, the substance that can form an acidic solution with weaker conductivity than \(\mathrm{NaCl}\) is the most likely candidate. - \(\mathrm{HNO}_{3}\): A strong acid, which will dissociate completely in water and produce a high concentration of ions, resulting in a relatively high conductivity. This does not match the description of the unknown solution. - \(\mathrm{H}_{3}\mathrm{PO}_{3}\): A weak acid, which will partially dissociate in water and produce a low concentration of ions, resulting in a relatively low conductivity. This aligns with the property of the unknown solution that states it has weaker conductivity than the \(\mathrm{NaCl}\) solution. It can be concluded that among the given substances, the unknown solution could be \(\mathrm{H}_{3}\mathrm{PO}_{3}\).

Key concepts

Acidic Solutions

When we say a solution is acidic, we mean that it has a higher concentration of hydrogen ions, \(\text{H}^{+}\), compared to pure water. This can change the color of indicators, like litmus paper, which turn red in the presence of an acid.
Acidic solutions are formed when certain substances dissolve in water and release these \(\text{H}^{+}\) ions.
These ion concentrations are crucial as they define the strength of an acid. Substances such as hydrochloric acid (\(\text{HCl}\)) or nitric acid (\(\text{HNO}_{3}\)) are known as strong acids because they completely dissociate in water. This means they release a large number of hydrogen ions, making the solution highly acidic.
In contrast, weak acids like phosphoric acid (\(\text{H}_{3}\text{PO}_{3}\)) only partially dissociate in water, releasing fewer hydrogen ions into the solution. This partial dissociation results in a lower concentration of hydrogen ions, thus, a less acidic solution.

• Strong acids = complete dissociation = high \(\text{H}^{+}\) ions.
• Weak acids = partial dissociation = low \(\text{H}^{+}\) ions.

When identifying an unknown acidic substance, examining its strength as an acid can be essential. In the given exercise, we focus on a solution's weak acidic nature, suggesting it might be a weak acid.

Conductivity of Solutions

Conductivity in solutions is the ability of the liquid to transport an electric current. This is due to the presence of free ions in the solution. Higher the number of these free-charge carriers, the better the conductivity. Let's break down what affects this property.
Strong acids and bases, such as hydrochloric acid or sodium chloride ( ext{NaCl}), completely dissociate in water, releasing plenty of ions and thus have high conductivity.
Meanwhile, weak acids like \(\text{H}_{3}\text{PO}_{3}\) only partly dissociate, releasing fewer ions. That's why they cause low conductivity, as was identified in the unknown solution of the exercise.

• Higher ion concentration = stronger conductivity.
• Lower ion concentration = weaker conductivity.

Understanding the conductivity helps determine how an ionic solution will behave. In some cases, it narrows down the potential candidates for an unknown solution; like in our exercise, leading us to \(\text{H}_{3}\text{PO}_{3}\) as it fits the weak conductivity criterion.

Ionization in Water

Ionization is the process by which a molecule splits into ions when placed in water. This process is crucial for understanding how different substances behave in aqueous solutions.
For acids, ionization means splitting into \(\text{H}^{+}\) ions and their conjugate bases. The extent of ionization is what classifies acids as either strong or weak.
In substances like nitric acid (\(\text{HNO}_{3}\)), ionization is complete: lots of \(\text{H}^{+}\) ions are produced, meaning less of the original acid remains intact. This complete ionization characterizes it as a strong acid.
On the other hand, weak acids like \(\text{H}_{3}\text{PO}_{3}\) only partially ionize. Most of the molecules remain as they are, with just a small proportion turning into ions. That’s why these are known as weak acids.

• Complete ionization = strong acids
• Partial ionization = weak acids

This concept of ionization helps us understand why some solutions conduct electricity well, and some do not. As seen in the given solution, the weak ionization of \(\text{H}_{3}\text{PO}_{3}\) aligns with the reduced conductivity observed.