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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 \(\mathrm{NaCl}\) 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

Expert verified
The unknown solute is \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (phosphorous acid) as it is acidic and has weak electrical conductivity compared to a \(\mathrm{NaCl}\) solution.

Step by step solution

01

Find Acidic or Basic Substances

First, we'll classify the given substances into acidic, basic, or neutral based on their chemical properties and formulas. 1. \(\mathrm{KOH}\): Potassium hydroxide is a strong base as it produces \(\mathrm{OH^{-}}\) ions in water. 2. \(\mathrm{NH}_{3}\): Ammonia is a weak base, which forms \(\mathrm{NH_{4}^{+}}\) and \(\mathrm{OH^{-}}\) ions in water. 3. \(\mathrm{HNO}_{3}\): Nitric acid is a strong acid, which produces \(\mathrm{H^{+}}\) ions in water. 4. \(\mathrm{KClO}_{2}\): Potassium chlorite is a salt, which is neither acidic nor basic but dissociates into \(\mathrm{K^{+}}\) and \(\mathrm{ClO_{2}^{-}}\) ions. 5. \(\mathrm{H}_{3} \mathrm{PO}_{3}\)): Phosphorous acid is a weak acid that produces \(\mathrm{H^{+}}\) ions in water. 6. \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (Acetone): Acetone is an organic compound and does not produce \(\mathrm{H^{+}}\) or \(\mathrm{OH^{-}}\) ions in water, so it's neutral. Out of these substances \(\mathrm{HNO}_{3}\), and \(\mathrm{H}_{3} \mathrm{PO}_{3}\) are acidic, but we need a solute that has weak electrical conductivity compared to \(\mathrm{NaCl}\).
02

Check for Electrical Conductivity

To distinguish between \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{3}\), we will analyze their ability to conduct electricity in an aqueous solution: 1. \(\mathrm{HNO}_{3}\): Nitric acid is a strong acid, meaning it completely dissociates in water to form many ions which increase the electrical conductivity of the solution. 2. \(\mathrm{H}_{3} \mathrm{PO}_{3}\): Phosphorous acid is a weak acid that partially dissociates in water, meaning it produces fewer ions and has less electrical conductivity compared to strong acid or salt solutions. Since we are looking for an acidic solute with weak electrical conductivity compared to a \(\mathrm{NaCl}\) solution, the unknown solute is \(\mathrm{H}_{3} \mathrm{PO}_{3}\).

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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 essentially a mixture where a solute is dissolved in water. Water acts as the solvent, the component that does the dissolving. Aqueous solutions can conduct electricity if the solute dissociates into ions. The extent of ionization of the solute greatly affects the solution's conductivity. In the original problem, an unknown solute is dissolved in water, creating an aqueous solution that is then tested for its acidity and electrical conductivity. The behavior of the solution in these tests can help identify the solute.
Weak Acid
A weak acid is characterized by its partial dissociation in water. This means only a few of its molecules release hydrogen ions ( H^+ ), which are responsible for the acidic nature of the solution. Weak acids can still affect the pH of an aqueous solution, but to a lesser degree than strong acids. Some common examples of weak acids are acetic acid and phosphorous acid ( H_3PO_3 ). In the exercise, since phosphorous acid is identified as a weak acid, it has a moderate acidic effect and less electrical conductivity than a strong acid would, due to fewer ions being present in the solution.
Electrical Conductivity
Electrical conductivity in a solution depends on the presence of ions, which act as charge carriers. The more ions present, the higher the conductivity. In an aqueous solution, strong acids or salts that fully dissociate into ions have high conductivity. In contrast, solutions with weak acids have lower conductivity due to the limited number of ions. This concept helps differentiate between HNO_3 , a strong acid, and H_3PO_3 , a weak acid, in the provided problem. The weakly conducting nature of the solution points to the presence of the weak acid, H_3PO_3 , rather than the strong acid, HNO_3 .
Litmus Test
The litmus test is a simple procedure used to determine whether a solution is acidic or basic. It involves using litmus paper, which changes color in response to the solution's pH. - Blue litmus paper turns red in acidic conditions. - Red litmus paper turns blue in basic conditions. In the original scenario, the unknown solute, when dissolved in water, turns the litmus paper red. This reaction indicates an acidic nature of the solution, thereby ruling out any bases in the list of potential solutes.
Chemical Properties
Chemical properties refer to the behavior of a substance when it undergoes a chemical change. These properties dictate how a substance reacts with other chemicals. Properties such as acidity, basicity, and the ability to conduct electricity are pivotal in determining the identity of an unknown solution. In this problem, the key chemical properties used to identify the solute are: - Acidity indicated by the litmus test. - Electrical conductivity showing how well the substance dissociates into ions. By understanding the chemical properties, the solute can be identified as H_3PO_3 , a weak acid with limited ionization in water.

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Most popular questions from this chapter

Identify the precipitate (if any) that forms when the following solutions are mixed, and write a balanced equation for each reaction. (a) \(\mathrm{NaCH}_{3} \mathrm{COO}\) and \(\mathrm{HCl}\), (b) \(\mathrm{KOH}\) and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\), (c) \(\mathrm{Na}_{2} \mathrm{~S}\) and \(\mathrm{CdSO}_{4}\).

We have learned in this chapter that many ionic solids dissolve in water as strong electrolytes; that is, as separated ions in solution. Which statement is most correct about this process? (a) Water is a strong acid and therefore is good at dissolving ionic solids. (b) Water is good at solvating ions because the hydrogen and oxygen atoms in water molecules bear partial charges. (c) The hydrogen and oxygen bonds of water are easily broken by ionic solids.

An 8.65-g sample of an unknown group \(2 \mathrm{~A}\) metal hydroxide is dissolved in \(85.0 \mathrm{~mL}\) of water. An acid-base indicator is added and the resulting solution is titrated with \(2.50 \mathrm{M}\) \(\mathrm{HCl}(a q)\) solution. The indicator changes color signaling that the equivalence point has been reached after \(56.9 \mathrm{~mL}\) of the hydrochloric acid solution has been added. (a) What is the molar mass of the metal hydroxide? (b) What is the identity of the metal cation: \(\mathrm{Ca}^{2+}, \mathrm{Sr}^{2+}, \mathrm{Ba}^{2+}\) ?

In each of the following pairs, indicate which has the higher concentration of I \({ }^{-}\)ion: (a) \(0.10 \mathrm{M} \mathrm{BaI}_{2}\) or \(0.25 \mathrm{M} \mathrm{KI}\) solution, (b) \(100 \mathrm{~mL}\) of \(0.10 \mathrm{M} \mathrm{KI}\) solution or \(200 \mathrm{~mL}\) of \(0.040 \mathrm{M} \mathrm{ZnI}\) solution, (c) \(3.2 \mathrm{M}\) HI solution or a solution made by dissolving \(145 \mathrm{~g}\) of \(\mathrm{NaI}\) in water to make \(150 \mathrm{~mL}\) of solution.

A fertilizer railroad car carrying 34,300 gallons of commercial aqueous ammonia ( \(30 \%\) ammonia by mass) tips over and spills. The density of the aqueous ammonia solution is \(0.88 \mathrm{~g} / \mathrm{cm}^{3}\). What mass of citric acid, \(\mathrm{C}(\mathrm{OH})(\mathrm{COOH})\left(\mathrm{CH}_{2} \mathrm{COOH}\right)_{2}\), (which contains three acidic protons) is required to neutralize the spill? 1 gallon \(=3.785 \mathrm{~L}\).
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