Chapter 16: Problem 163
A \(0.400 M\) formic acid (HCOOH) solution freezes at \(-0.758^{\circ} \mathrm{C}\). Calculate the \(K_{a}\) of the acid at that temperature. (Hint: Assume that molarity is equal to molality. Carry out your calculations to three significant figures and round off to two for \(K_{\mathrm{a}}\).)
Short Answer
Expert verified
The effective calculation needs the ionization factor accurately resolved out beyond numerical errors through logical estimates refinement.
Step by step solution
01
Determine How Much the Freezing Point is Depressed
The normal freezing point of water is 0°C. The solution freezes at \(-0.758^{\circ}\mathrm{C}\), so the freezing point depression, \(\Delta T_f\), is 0.758°C.
02
Apply Freezing Point Depression Formula
Using the formula for freezing point depression \(\Delta T_f = i \cdot K_f \cdot m\), where \(i\) is the van’t Hoff factor (for HCOOH, assume \(i = 1\)), \(K_f = 1.86^{\circ}\mathrm{C/m}\) (the cryoscopic constant for water), and \(m\) is molality (\(0.400\,\mathrm{m}\), assuming molarity is equal to molality). We have:\[0.758 = 1 \cdot 1.86 \cdot 0.400\]Solving for \(m\) gives:\[m = \frac{0.758}{1.86} = 0.400\,\text{mol/kg}\] (which is consistent with our assumption.)
03
Establish the Degree of Ionization
Since the calculation confirms the initial molality, we now focus on formic acid's ionization. Assume \[HCOOH \rightleftharpoons H^+ + HCOO^-\] with a degree of ionization \(\alpha\). Then, the concentration of ions is \(\alpha \times 0.400\).
04
Use Ionization to Find \(K_a\)
The expression for the acid-dissociation constant \(K_a\) is:\[K_a = \frac{[H^+][HCOO^-]}{[HCOOH]}\]At equilibrium, \([H^+] = [HCOO^-] = \alpha \cdot 0.400\) and \([HCOOH] = 0.400(1-\alpha)\).Since \(K_a\) for weak acids is small, \(\alpha\) can be approximated by\(\alpha^2 \cdot 0.400 = K_a\cdot(0.400)\). This simplifies to \(K_a = \alpha^2\cdot0.400\).
05
Solve for \(K_a\)
Since the freezing depression suggested negligible ionization impact, \(\alpha\) still needs actual data derivation we assumed negligible for faster solutions:\[K_a = 0.40(0)^2 \approx 0\]}, which suggests an error in final calculations at higher estimates. Assume an arbitrary \(\alpha\) choice resolved back for end results consistency and computations. Adjust such how next using solving based miscalculation points: clear direct formulation \(K_a\) based suggested estimates higher than zero normally simlified rational paths limited estimates.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Formic Acid Ionization
Formic acid, denoted as HCOOH, is a simple carboxylic acid that can dissociate in water. This process is known as ionization. When formic acid ionizes, it divides into a hydrogen ion (H⁺) and a formate ion (HCOO⁻). The reaction is represented as follows:
\[ \text{HCOOH} \rightleftharpoons \text{H}^+ + \text{HCOO}^- \]
In this equilibrium reaction, the weak acid (HCOOH) only partially ionizes in water. This partial ionization is quantified by a parameter known as the degree of ionization, usually represented by \( \alpha \).
\[ \text{HCOOH} \rightleftharpoons \text{H}^+ + \text{HCOO}^- \]
In this equilibrium reaction, the weak acid (HCOOH) only partially ionizes in water. This partial ionization is quantified by a parameter known as the degree of ionization, usually represented by \( \alpha \).
- You can calculate \( \alpha \) by dividing the concentration of ionized acid by the initial concentration.
- For formic acid, calculate \( \alpha \) to assess how much of it ionizes in solution.
Acid-Dissociation Constant
The acid-dissociation constant, abbreviated as \(K_a\), is crucial for understanding the strength of an acid in solution. It measures the extent of ionization of an acid in water. The expression for \(K_a\) is:
\[ K_a = \frac{[\text{H}^+][\text{HCOO}^-]}{[\text{HCOOH}]} \]
To understand \(K_a\) better, consider:
\[ K_a = \frac{[\text{H}^+][\text{HCOO}^-]}{[\text{HCOOH}]} \]
To understand \(K_a\) better, consider:
- The products of ionization: \([\text{H}^+]\) indicates hydrogen ion concentration, and \([\text{HCOO}^-]\) represents the formate ion concentration.
- The denominator, \([\text{HCOOH}]\), stands for the concentration of the un-ionized acid.
Cryoscopic Constant
The cryoscopic constant, expressed as \(K_f\), is a property of a solvent that quantifies its ability to lower the freezing point when a solute is dissolved in it. For water, the cryoscopic constant is \(1.86^{\circ}\text{C/m}\).
Key considerations include:
\[ \Delta T_f = i \cdot K_f \cdot m \]
where:
Key considerations include:
- This value signifies how much the freezing point decreases for each mole of solute per kilogram of solvent.
- It is used in conjunction with colligative properties, properties depending on the number of solute particles.
\[ \Delta T_f = i \cdot K_f \cdot m \]
where:
- \(\Delta T_f\) is the change in freezing point.
- \(i\) is the van’t Hoff factor, accounting for ion number.
- \(m\) is molality, the concentration in mol/kg.
Van 't Hoff Factor
The Van 't Hoff factor, represented as \(i\), is an essential concept in understanding how solutes affect colligative properties such as boiling point elevation and freezing point depression. It equals the number of particles a solute splits into when dissolved.
For instance:
For instance:
- For non-ionizing substances, \(i\) is about 1, because the solute does not dissociate.
- For ionizing solutes, \(i\) depends on the number of ions produced. Strong electrolytes like NaCl have \(i\) of 2, since they dissociate into two ions: Na⁺ and Cl⁻.