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Ch.17 - Aqueous Ionic Equilibrium
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831당신이 사용하는 게 아니라요?교과서 변경
모든 교과서Tro 4th EditionCh.17 - Aqueous Ionic Equilibrium문제 136
17장, 문제 136

You are asked to prepare 2.0 L of an HCN/NaCN buffer that has a pH of 9.8 and an osmotic pressure of 1.35 atm at 298 K. What masses of HCN and NaCN should you use to prepare the buffer? (Assume complete dissociation of NaCN.)

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Identify the relevant equations: Use the Henderson-Hasselbalch equation for buffer solutions: \( \text{pH} = \text{pK}_a + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where \([\text{A}^-]\) is the concentration of the conjugate base (NaCN) and \([\text{HA}]\) is the concentration of the weak acid (HCN).
Determine the \(\text{pK}_a\) of HCN: Look up the \(\text{pK}_a\) value for HCN, which is approximately 9.21.
Calculate the ratio \( \frac{[\text{A}^-]}{[\text{HA}]} \) using the Henderson-Hasselbalch equation: Rearrange the equation to find \( \frac{[\text{A}^-]}{[\text{HA}]} = 10^{(\text{pH} - \text{pK}_a)} \).
Use the osmotic pressure equation to find the total molarity: The osmotic pressure equation is \( \Pi = iMRT \), where \( \Pi \) is the osmotic pressure, \( i \) is the van't Hoff factor (which is 2 for NaCN due to complete dissociation), \( M \) is the molarity, \( R \) is the ideal gas constant (0.0821 L·atm/mol·K), and \( T \) is the temperature in Kelvin.
Solve for the individual concentrations of HCN and NaCN: Use the total molarity from the osmotic pressure equation and the ratio from the Henderson-Hasselbalch equation to set up a system of equations. Solve for \([\text{HCN}]\) and \([\text{NaCN}]\), then convert these concentrations to masses using their respective molar masses.

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Buffer Solutions

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, HCN (a weak acid) and NaCN (its conjugate base) form a buffer that can maintain a pH of 9.8.
추천 영상:
가이드 코스
03:02
Buffer Solutions

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the concentration of its acid and conjugate base. It is expressed as pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant. This equation is essential for calculating the required concentrations of HCN and NaCN to achieve the desired pH.
추천 영상:
가이드 코스
02:40
Henderson-Hasselbalch Equation

Osmotic Pressure

Osmotic pressure is the pressure required to prevent the flow of solvent into a solution through a semipermeable membrane. It can be calculated using the formula π = iCRT, where π is the osmotic pressure, i is the van 't Hoff factor, C is the molarity of the solution, R is the ideal gas constant, and T is the temperature in Kelvin. Understanding osmotic pressure is crucial for determining the concentrations of HCN and NaCN needed to achieve the specified osmotic pressure of 1.35 atm.
추천 영상:
가이드 코스
01:13
Osmotic Pressure Formula
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