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Chapter 4: Problem 36

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 NaCl of the same concentration. Which of the following substances could the unknown be: 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

Expert verified
The unknown substance is likely to be H鈧働O鈧� (Phosphorous acid) as it forms an acidic solution and has weak conductivity when dissolved in water, which matches the given description.

Step by step solution

01

Identify acidic substances

First, we'll determine which of the given substances can produce acidic solutions when dissolved in water. Acids donate protons (H鈦�) into the solution, resulting in a lowered pH. 1. KOH - This is a strong base, which would produce a basic solution, not acidic. 2. NH鈧� - This is a weak base and can't form an acidic solution. 3. HNO鈧� - This is a strong acid and can form an acidic solution. 4. KClO鈧� - This is a salt, and it doesn't have any acidic protons so it cannot form an acidic solution. 5. H鈧働O鈧� - This is a weak acid, and it can form an acidic solution. 6. CH鈧僀OCH鈧� (acetone) - This is a neutral organic compound, and it does not form acidic solutions. From the above list, HNO鈧� and H鈧働O鈧� are the only substances that can form acidic solutions. Now we need to analyze their conductivity properties.
02

Compare conductivity between substances

Now let's analyze the conductivity of the remaining two substances: 1. HNO鈧�: This is a strong acid that dissociates completely in water into H鈦� and NO鈧冣伝 ions. This results in a high concentration of ions in the solution, which would make it a good conductor. But the unknown solution is weakly conducting, so HNO鈧� cannot be the unknown substance. 2. H鈧働O鈧�: This is a weak acid that does not completely dissociate in water. Its dissociation generates a lower concentration of ions in the solution and results in weak conductivity. This matches the description of the unknown substance. Considering both the acidic nature and conductivity, the unknown substance is likely to be: H鈧働O鈧� (Phosphorous acid)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Aqueous Solution
An aqueous solution is simply a solution where water serves as the solvent. In such solutions, the solute鈥攖he substance being dissolved鈥攏egotiates with water molecules to form a homogeneous mixture. This process depends heavily on the nature of the solute: whether it's ionic or covalent. Aqueous solutions are integral to many chemical reactions and processes, as water is an excellent solvent, often helping to dissolve a wide range of substances.
  • Water acts as a solvent in which solute particles are dispersed.
  • Ionic compounds tend to dissociate in aqueous solutions.
  • Covalent substances may dissolve without dissociation.
Understanding how different substances behave in aqueous solutions is crucial in predicting a solution's characteristics, such as its acidity or conductivity.
Conductivity
Conductivity in chemistry refers to a solution's ability to conduct electricity. This property is directly linked to the presence of free ions in the solution. When ionic compounds dissolve in water, they separate into charged particles called ions. These ions carry electrical current through the solution.
  • High conductivity indicates a high concentration of ions.
  • Weakly conducting solutions have fewer ions present.
A weak acid, like phosphorous acid, dissociates marginally, contributing fewer ions compared to strong acids like nitric acid. Thus, the conductivity test becomes a valuable tool for identifying properties of substances in aqueous solutions.
Acid Dissociation
Acid dissociation is the process by which an acid molecule splits into ions when dissolved in water. This process is fundamental to understanding acids' behavior in solutions. Strong acids, such as nitric acid, dissociate completely, resulting in a high number of hydrogen ions, which greatly affect the solution's properties, including its pH and conductivity.
  • Complete dissociation: Results in strong conductivity.
  • Partial dissociation: Leads to weak conductivity.
  • Dissociation is crucial to chemical reactions involving acids.
    • For weak acids like phosphorous acid, only a fraction of acid molecules dissociate, which explains both its acidic nature and low conductivity in solutions.
    Weak Acid
    A weak acid is one that doesn't fully dissociate into its ions when in an aqueous solution. This means that the concentration of hydrogen ions ( H鈦� ) in the solution is lower compared to strong acids, leading to a higher pH. Weak acids often serve important roles in buffers, regulating pH in a variety of chemical and biological processes.
    • Incomplete dissociation characterizes weak acids.
    • Examples include phosphorous acid and acetic acid.
    • Important in biological and chemical systems due to buffering properties.
    In the context of the exercise, the weak conductivity exhibited by the unknown solution points towards phosphorous acid, a representative weak acid.
    Litmus Test
    The litmus test is a simple, quick way to determine the acidity or basicity of a solution. Litmus paper is soaked in dyes derived from lichens, which change color according to the pH of the solution they are in. When immersed in an acidic solution, litmus paper will turn red, indicating the presence of hydrogen ions. This test doesn't provide the strength of the acid but confirms its presence.
    • Red indicates acidity (pH less than 7).
    • Blue indicates basicity (pH greater than 7).
    Within the context of the exercise, the litmus test helped identify the unknown solution as acidic, helping narrow down the possibilities to those with acidic characteristics, eventually pointing towards phosphorous acid due to its weak conductivity and acidic nature.

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    Most popular questions from this chapter

    Magnesium carbonate, magnesium oxide, and magnesium hydroxide are all white solids that react with acidic solutions. (a) Write a balanced molecular equation and a netionic equation for the reaction that occurs when each substance reacts with a hydrochloric acid solution. (b) By observing the reactions in part (a), how could you distinguish any of the three magnesium substances from the other two?

    Using the activity series (Table 4.5), write balanced chemical equations for the following reactions. If no reaction occurs, write \(\mathrm{NR}\) . (a) Nickel metal is added to a solution of copper(II) nitrate, (b) a solution of zinc nitrate is added to a solution of magnesium sulfate, (c) hydrochloric acid is added to gold metal, (d) chromium metal is immersed in an aqueous solution of cobalt(II) chloride, (e) hydrogen gas is bubbled through a solution of silver nitrate.

    Consider the following reagents: zinc, copper, mercury (density 13.6 \(\mathrm{g} / \mathrm{mL}\) , silver nitrate solution, nitric acid solution. (a) Given a 500 -mL Erlenmeyer flask and a balloon, can you combine two or more of the foregoing reagents to initiate a chemical reaction that will inflate the balloon? Write a balanced chemical equation to represent this process. What is the identity of the substance that inflates the balloon? (b) What is the theoretical yield of the substance that fills the balloon? (c) Can you combine two or more of the foregoing reagents to initiate a chemical reaction that will produce metallic silver? Write a balanced chemical equation to represent this process. What ions are left behind in solution? (d) What is the theoretical yield of silver?

    One cup of fresh orange juice contains 124 \(\mathrm{mg}\) of ascorbic acid (vitamin \(\mathrm{C}, \mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{6} ) .\) Given that one cup \(=236.6 \mathrm{mL}\) calculate the molarity of vitamin \(\mathrm{C}\) in orange juice.

    Acetone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3},\) is a nonelectrolyte; hypochlorous \(\mathrm{acid}, \mathrm{HClO}\) , is a weak electrolyte; and ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl},\) is a strong electrolyte. (a) What are the solutes present in aqueous solutions of each compound? (b) If 0.1 mol of each compound is dissolved in solution, which one contains 0.2 mol of solute particles, which contains 0.1 mol of solute particles, and which contains somewhere between 0.1 and 0.2 mol of solute particles?
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