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 substance could be \(\mathrm{H}_{3} \mathrm{PO}_{3}\), as it has both acidic nature and weak conductivity compared to a \(\mathrm{NaCl}\) solution of the same concentration.

Step-by-step solution

Check if KOH is the unknown acidic substance

\(\mathrm{KOH}\), or potassium hydroxide, is a strong base when dissolved in water. It dissociates fully into the ions \(\mathrm{K}^{+}\) and \(\mathrm{OH}^{-}\), which causes the solution to be alkaline, not acidic. Thus, \(\mathrm{KOH}\) cannot be the unknown substance.

Check if NH3 is the unknown acidic substance

\(\mathrm{NH}_{3}\), or ammonia, is a weak base when dissolved in water because it forms the ammonium ion \(\mathrm{NH}_{4}^{+}\) and a hydroxide ion \(\mathrm{OH}^{-}\). This makes the solution slightly alkaline, not acidic. So, \(\mathrm{NH}_{3}\) cannot be the unknown substance.

Check if HNO3 is the unknown acidic substance

\(\mathrm{HNO}_{3}\), or nitric acid, is a strong acid, which means that it dissociates fully into \(\mathrm{H}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions. While its acidic nature agrees with the litmus paper result, the high concentration of ions would make the solution strongly conducting compared to the given \(\mathrm{NaCl}\) solution. Hence, \(\mathrm{HNO}_{3}\) is not the unknown substance.

Check if KClO2 is the unknown acidic substance

\(\mathrm{KClO}_{2}\) is a salt that dissociates into \(\mathrm{K}^{+}\) and \(\mathrm{ClO}_{2}^{-}\) ions. Neither of these ions would produce acidic or basic properties, so the solution remains neutral. Therefore, \(\mathrm{KClO}_{2}\) cannot be the unknown substance.

Check if H3PO3 is the unknown acidic substance

\(\mathrm{H}_{3} \mathrm{PO}_{3}\), or phosphorous acid, is a weak acid that partially dissociates in water to produce \(\mathrm{H}^{+}\) ions. The acidic nature of the solution matches the litmus paper result, and its weak conductivity is due to the partial dissociation of the solute. Thus, \(\mathrm{H}_{3} \mathrm{PO}_{3}\) could be the unknown substance.

Check if CH3COCH3 is the unknown acidic substance

\(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), or acetone, is a neutral organic compound. When dissolved in water, it does not produce any acidic or basic properties. Thus, acetone cannot be the unknown substance. In conclusion, considering the acidic nature and weak conductivity of the solution, the unknown substance could be \(\mathrm{H}_{3} \mathrm{PO}_{3}\).

Key concepts

Acid-Base Reactions

In the realms of chemistry, acid-base reactions are fundamental. They refer to the chemical processes where an acid and a base interact, often resulting in the formation of water and a salt. Acids are substances that can donate a proton (\textbf{H}^+), and bases are substances that can accept a proton. This concept helps us decipher why \textbf{NH}\(_3\), known as ammonia, and \textbf{KOH}, potassium hydroxide, were not the mysterious acidic substance in our exercise. Ammonia is a weak base that can accept \textbf{H}^+ ions, while potassium hydroxide is a strong base that provides \textbf{OH}^− ions into the solution.

Understanding that acids react with bases to neutralize each other provides valuable insight into predicting the outcomes of chemical reactions. For instance, when examining \textbf{H}\(_3\)\textbf{PO}\(_3\), or phosphorous acid, a weak acid capable of donating protons is revealed. Its inability to fully dissociate in water aligns with the weak electrical conductivity observed in the exercise. Thus, through understanding acid-base reactions, we can identify that phosphorous acid fits the criteria of a weak, partially dissociating acid responsible for the acidic solution in question.

Electrolyte Conductivity

Electrolyte conductivity is a measure of a substance's ability to conduct electricity when dissolved in water. This property is determined by the presence of free ions in the solution. Strong acids and bases, like \textbf{HNO}\(_3\) (nitric acid) and \textbf{KOH} (potassium hydroxide), fully dissociate into ions, resulting in a high conductivity. On the other hand, weak acids such as \textbf{H}\(_3\)\textbf{PO}\(_3\), do not completely dissociate, thus exhibiting lower conductivity.

In the exercise, the unknown substance displayed weak conductivity when compared to a \textbf{NaCl} solution, indicating it was not a strong electrolyte. These details help us rule out strong acids and bases, steering the conclusion towards phosphorous acid, which aligns with both the acidic test result and the observed weak electrolyte conductivity.

pH and Litmus Paper Test

The pH scale quantifies the acidity or alkalinity of a solution. Ranging from 0 to 14, with 7 as neutral, numbers below 7 indicate acidic solutions, and those above 7 point to alkalinity. The pH is determined by the concentration of hydrogen ions (\textbf{H}^+) in the solution. The litmus paper test is a quick method to determine this characteristic; blue litmus turns red in acidic conditions, while red litmus paper turns blue in basic conditions.

In our exercise, the litmus paper turned red upon contact with the unknown solution, confirming its acidic nature. This simple test is instrumental in identifying the pH level qualitatively. Thus, the combination of a litmus paper reading with knowledge of electrolyte conductivity and acid-base properties allowed us to deduce that the unknown acidic, weakly conducting substance was likely phosphorous acid, \textbf{H}\(_3\)\textbf{PO}\(_3\).