Question

You mix \(30.0 \mathrm{mL}\) of \(0.15 \mathrm{M} \mathrm{NaOH}\) with \(30.0 \mathrm{mL}\) of \(0.15 \mathrm{M}\) acetic acid. What molecules and ions exist in this solution? List them in order of decreasing concentration.

Short answer

Water, \( \text{CH}_3\text{COO}^- \), \( \text{Na}^+ \), \( \text{CH}_3\text{COOH} \), in decreasing concentration.

Step-by-step solution

Identify Major Reactants

First, let's determine the major components of the solution. We have sodium hydroxide \( \text{NaOH} \) and acetic acid \( \text{CH}_3\text{COOH} \). Both are present in the solution in equal volumes and molarity.

Balanced Reaction Equation

Sodium hydroxide \( \text{NaOH} \) is a strong base, and acetic acid \( \text{CH}_3\text{COOH} \) is a weak acid. They react according to the equation: \[\text{NaOH} + \text{CH}_3\text{COOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}\] This means that the base will neutralize the acid, forming sodium acetate \( \text{CH}_3\text{COONa} \) and water.

Determine Limiting Reactant and Reaction Progress

Equal volumes (30 mL) and molarities (0.15 M) imply that both acetic acid and sodium hydroxide are present in equimolar amounts. Therefore, they completely react with each other, as neither is in excess.

Final Components in Solution

Post-reaction, the major species in the solution are the ions from the completely dissociated products and any excess neutral molecules. We have:1. \( \text{CH}_3\text{COONa} \), which dissociates into \( \text{CH}_3\text{COO}^- \) and \( \text{Na}^+ \).2. Water \( \text{H}_2\text{O} \) from the reaction.3. Since \( \text{NaOH} \) reacted completely, \( \text{OH}^- \) is not present.4. \( \text{CH}_3\text{COOH} \) is weak and thus mostly exists as molecules.

Order by Concentration

The concentrations of species in solution, from highest to lowest, will be:1. Water molecules \( \text{H}_2\text{O} \) since it is the solvent.2. \( \text{CH}_3\text{COO}^- \) ions from dissociation of \( \text{CH}_3\text{COONa} \).3. \( \text{Na}^+ \) ions, also from \( \text{CH}_3\text{COONa} \).4. Neutral acetic acid molecules \( \text{CH}_3\text{COOH} \), which are undissociated.

Key concepts

Sodium Acetate Formation

In an acid-base reaction, sodium hydroxide (\( \text{NaOH} \)), a strong base, reacts with acetic acid (\( \text{CH}_3\text{COOH} \)), a weak acid, to form sodium acetate (\( \text{CH}_3\text{COONa} \)).When \( \text{NaOH} \) and \( \text{CH}_3\text{COOH} \) are mixed, they undergo a neutralization reaction.The balanced chemical equation for this reaction is: \[ \text{NaOH} + \text{CH}_3\text{COOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} \]This reaction results in the formation of sodium acetate and water (\( \text{H}_2\text{O} \)).Sodium acetate is a salt that dissociates into sodium ions \( \text{Na}^+ \) and acetate ions \( \text{CH}_3\text{COO}^- \).
- The presence of sodium acetate in the solution is significant because it impacts the pH and behavior of the solution.It serves as a source of acetate ions, which can further interact with hydrogen ions (\( \text{H}^+ \)), influencing the acidity or basicity of the solution.

Understanding Weak Acids

Acetic acid (\( \text{CH}_3\text{COOH} \)) is a classic example of a weak acid.Weak acids only partially dissociate into their ions in a solution.This means that in a solution of acetic acid, you have a mix of both acetic acid molecules and acetate ions (\( \text{CH}_3\text{COO}^- \)).
For weak acids:

• Their molecular form remains the primary component because dissociation does not go to completion.
• The dissociation equilibrium is represented by: \[ \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \]

This equilibrium affects how acetic acid behaves in the solution.Since it is a weak acid, its presence in large amounts primarily as undissociated molecules influences the solution's pH.Thus, even after reaction, some acetic acid remains undissociated amongst the products in the solution.

Neutralization Reaction

A neutralization reaction is a chemical reaction in which an acid and a base react to form a salt and water.In this case, acetic acid (\( \text{CH}_3\text{COOH} \)) reacts with sodium hydroxide (\( \text{NaOH} \)), resulting in sodium acetate (\( \text{CH}_3\text{COONa} \)) and water.
The equation for this is:\[\text{NaOH} + \text{CH}_3\text{COOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} \]This reaction effectively neutralizes the acid and removes any unreacted base.The neutralization reaction proceeds until the limiting reactant is completely consumed.

• Since both the sodium hydroxide and the acetic acid are present in equal molar amounts, neither substance is left in excess after the reaction.
• This allows sodium acetate to be produced without any leftover starting ingredients, confirming the expected outcome in the balanced reaction.

Understanding neutralization reactions helps in comprehending the final composition of the solution and how the different ions and molecules interact within.