Question

A typical solution used in general chemistry laboratories is 3.0 M HCl. Describe, in detail, the composition of 2.0 L of a 3.0-M HCl solution. How would 2.0 L of a \(3.0-M \mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) solution differ from the same quantity of the HCl solution?

Short answer

The composition of 2.0 L of a 3.0 M HCl solution consists of 6.0 moles of HCl dissolved in water, where HCl completely dissociates into 6.0 moles of H+ ions and 6.0 moles of Cl- ions. Comparatively, 2.0 L of a 3.0 M HC2H3O2 (acetic acid) solution also contains 6.0 moles of solute; however, it does not completely dissociate due to its weak acidic nature, resulting in fewer H+ and C2H3O2- ions. This makes the HCl solution more acidic than the HC2H3O2 solution.

Step-by-step solution

Find the number of moles of solute in each solution

To find the number of moles of solute in a solution, we can use the equation: moles = Molarity * Volume. For the 3.0 M HCl solution: moles of HCl = Molarity * Volume moles of HCl = 3.0 mol/L * 2.0 L moles of HCl = 6.0 mol For the 3.0 M HC2H3O2 solution: moles of HC2H3O2 = Molarity * Volume moles of HC2H3O2 = 3.0 mol/L * 2.0 L moles of HC2H3O2 = 6.0 mol Now that we know the number of moles of solute in each solution, we can compare their compositions.

Describe the composition of the 3.0 M HCl solution

A 2.0 L solution of 3.0 M HCl contains 6.0 moles of HCl dissolved in enough water to make 2.0 L of the solution. Hydrochloric acid (HCl) is a strong acid, meaning that it completely dissociates (separates) into its constituent ions (H+ and Cl-) in solution. In this case, we would have 6.0 moles of H+ ions and 6.0 moles of Cl- ions in the 2.0 L solution.

Describe the composition of the 3.0 M HC2H3O2 solution

A 2.0 L solution of 3.0 M HC2H3O2 (acetic acid) contains 6.0 moles of HC2H3O2 dissolved in enough water to make 2.0 L of the solution. Acetic acid is a weak acid, which means that it does not completely dissociate into its constituent ions (H+ and C2H3O2-) in the solution. Thus, in this case, there will be fewer than 6.0 moles of H+ ions, and fewer than 6.0 moles of C2H3O2- ions in the solution, with some HC2H3O2 molecules remaining undissociated (intact).

Compare the two solutions

Based on the above analysis, we can see that both 3.0 M HCl and 3.0 M HC2H3O2 solutions contain 6.0 moles of solute in 2.0 L of water. In the HCl solution, all 6.0 moles of HCl dissociate into H+ and Cl- ions. However, in the HC2H3O2 solution, not all 6.0 moles of acetic acid dissociate into H+ and C2H3O2- ions due to its weak acidic nature. As a result, the HCl solution would have a higher concentration of H+ ions, making it more acidic than the HC2H3O2 solution.

Key concepts

Molarity

Molarity is a key concept in chemistry used to describe the concentration of a solution. It is defined as the number of moles of solute per liter of solution. The formula used to calculate molarity is:\[\text{Molarity} (M) = \frac{\text{moles of solute}}{\text{liters of solution}}\]This mathematical expression helps us understand how much solute is present in a particular volume of a solution, which is crucial for reactions and chemical processes.
For instance, in the given exercise, both the Hydrochloric Acid (HCl) and Acetic Acid (HC₂H₃O₂) solutions have a molarity of 3.0 M. This tells us they contain 3.0 moles of solute in every liter of solution. Therefore, in 2.0 liters of solution, each contains 6.0 moles of their respective solute. Understanding molarity allows chemists to control reactant concentrations in reactions, predict outcomes, and ensure proper chemical behavior.
Always remember, to increase the molarity, you can either increase the amount of solute or decrease the volume of the solution. Conversely, to decrease molarity, reduce the solute or increase the solution's volume.

• Moles of Solute: Indicates quantity of chemical species.
• Liters of Solution: Represents the solution's volume.
• Makes it easier to calculate solutions needed for reactions.

Hydrochloric Acid

Hydrochloric Acid, or HCl, is a strong acid commonly used in various laboratory and industrial applications. What makes HCl a strong acid is its complete dissociation in water. This means that when dissolved, it fully separates into hydrogen ions (H\(^+\)) and chloride ions (Cl\(^-\)). This characteristic is due to the strength of the H-Cl bond and the ease with which it ionizes in water.
In the context of the given solution, a 2.0 liter of 3.0 M HCl solution contains 6.0 moles of HCl. Once dissociated, this results in 6.0 moles of H\(^+\) ions and 6.0 moles of Cl\(^-\) ions. This high concentration of H\(^+\) ions makes the HCl solution very acidic.
Here are some key points about HCl:

• Known as muriatic acid in its industrial form.
• Widely used for pH adjustments and cleaning purposes.
• Demands careful handling due to its corrosive nature.

Because of its complete dissociation, Hydrochloric Acid solutions are very efficient at providing a strong acidic environment, useful in a wide array of chemical reactions.

Acetic Acid

Acetic Acid (HC\(_{2}\)H\(_{3}\)O\(_{2}\)) is a well-known weak acid found in substances like vinegar. Unlike strong acids, weak acids do not completely dissociate in solution. In a 3.0 M acetic acid solution, only a fraction of the acetic acid molecules ionizes to form hydrogen ions (H\(^+\)) and acetate ions (C\(_{2}\)H\(_{3}\)O\(_{2}\)\(^{-}\)).
This partial dissociation means that there will be fewer hydrogen ions in the solution compared to a strong acid solution of the same molarity. In the exercise described, even though there are 6.0 moles of Acetic Acid present in 2.0 liters of solution, not all of these will contribute to free hydrogen ions. A portion will remain as undissociated HC\(_{2}\)H\(_{3}\)O\(_{2}\) molecules in the solution.
Here's what to keep in mind about Acetic Acid:

• Commonly used in food as vinegar, containing about 4-8% acetic acid by volume.
• Serves as a chemical reagent and industrial solvent.
• The weak acidic nature makes it less corrosive compared to strong acids like HCl.

The distinction between strong and weak acids such as Acetic Acid is vital, especially in chemical reactions where the concentration of hydrogen ions significantly impacts the reaction's behavior and outcomes.