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 could be H₃PO₃.

Step-by-step solution

Understand the Problem

We need to identify a substance that is acidic and weakly conducting compared to NaCl. This requires us to consider the basic or acidic nature of each substance and its ionization properties.

Analyze the Properties of Each Substance

Let's review the nature of each substance in relation to acids, bases, and conductivity: 1. **KOH**: Strong base, fully ionizes in water. 2. **NH₃**: Weak base, does not produce an acidic solution. 3. **HNO₃**: Strong acid, fully ionizes in water. 4. **KClO₂**: Salt, neutral in solution. 5. **H₃PO₃**: Weak acid, partially ionizes in water. 6. **CH₃COCH₃**: Neutral organic solvent, does not produce ions in solution.

Evaluate the Litmus Test Result

The solution changes litmus paper to indicate acidity. This means the unknown substance must be an acid or have acidic properties.

Consider Conductivity

The solution is weakly conducting. This suggests the substance does not fully ionize in water, pointing towards a weak acid or base, but doesn't support strong acids or ionic salts which fully dissociate.

Identify the Substance

Given the criteria of being acidic and weakly conducting, **H₃PO₃** is the most likely candidate. It is a weak acid and does not fully ionize in water, resulting in lower conductivity compared to NaCl.

Key concepts

Weak Acids

Weak acids are fascinating players in the world of chemistry. Unlike strong acids, weak acids only partially ionize in water. This means that not all of their molecules release hydrogen ions (H⁺) when dissolved. A common example of a weak acid is phosphorous acid, (H₃PO₃). In a water solution, only a fraction of the H₃PO₃ molecules donate their hydrogen ions, making the acidic solution less concentrated in H⁺ ions.

Here are some characteristics of weak acids:

• Partial ionization: Only some of the acid molecules dissociate in solution, producing fewer ions.
• Lower pH compared to strong acids, but not as low.
• Weak electrolyte: Their solutions do not conduct electricity as well.

Remember, the key point is that weak acids like phosphorous acid do not completely break apart into ions in solutions, which influences their behavior and their programs of use in chemical reactions.

Conductivity

Conductivity refers to the ability of a solution to conduct electricity. When substances dissolve in water and dissociate into ions, they can carry an electrical current, making the solution conductive. As you might guess, the more ions present, the better the conduction!

In solutions where a weak acid is dissolved, you will find that their ability to conduct electricity is much less than that of strong acids or ionic compounds like NaCl. Here's why:

• Fewer free-moving charged particles (ions) mean less ability to carry electrical current.
• Weak acids, which partially ionize, create fewer ions compared to strong acids or ionic compounds.

For example, this is why an aqueous solution of phosphorous acid is weakly conducting - (H₃PO₃) creates fewer ions as it doesn't fully ionize in water.

Ionization

Ionization is the process by which a molecule splits into its component ions when dissolved in water. In the case of acids, this means releasing hydrogen ions (H⁺) into solution.

Full ionization happens with strong acids because they completely dissociate into ions. In contrast, weak acids like H₃PO₃ only partially ionize. This partial ionization results in a smaller concentration of H⁺ ions, thus impacting the solution's properties such as pH and conductivity.

Here’s what you should know about ionization of weak acids:

• Partial ionization limits the number of free hydrogen ions.
• This partial ionization also affects how such solutions react with bases and their overall acidity.
• Lower degree of ionization means lower conductivity and higher pH than strong acids.

Understanding ionization is crucial because it underscores why solutions of weak acids behave differently, not only in terms of acidity but also in their electrical conduction properties.

pH Testing

pH testing provides a straightforward way to determine the acidity of a solution. It is a measure of how many free hydrogen ions (H⁺) are in a solution, with lower pH values indicating more acidic solutions.

When using pH testing to identify weak acids, you'll usually find that their pH values are higher compared to strong acids, yet still lower than neutral substances. Here's how it works:

• Weak acids like H₃PO₃, when dissolved in water, produce some hydrogen ions, lowering the pH of the solution.
• The pH of a weak acid solution is typically between 4 and 6.
• Litmus or pH paper can easily indicate this acidic nature by changing color.

This simple test can give us a wealth of information about the acidic nature of a solution and help infer whether the acid is strong or weak based on its pH reading.