You are asked to prepare a pH = 3.00 buffer solution starting from 1.25 L of a 1.00 M solution of hydrofluoric acid (HF) and any amount you need of sodium fluoride (NaF). (b) How many grams of sodium fluoride should be added to prepare the buffer solution? Neglect the small volume change that occurs when the sodium fluoride is added.

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Calculate the initial moles of HF in the solution by multiplying the concentration of HF by the volume of the solution in liters.

Use the Henderson-Hasselbalch equation for buffer solutions, pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the fluoride ion (F-) and [HA] is the concentration of hydrofluoric acid (HF). The pKa of HF is approximately 3.17.

Rearrange the Henderson-Hasselbalch equation to solve for the ratio [F-]/[HF] needed to achieve the desired pH of 3.00.

Calculate the moles of F- needed based on the moles of HF initially present and the ratio [F-]/[HF] obtained from the Henderson-Hasselbalch equation.

Convert the moles of F- to grams of sodium fluoride (NaF) by using the molar mass of NaF (approximately 42 g/mol).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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, hydrofluoric acid (HF) acts as the weak acid, while sodium fluoride (NaF) provides the conjugate base, fluoride ion (F-), necessary to maintain the desired pH.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a mathematical formula used to calculate the pH of a buffer solution. It is expressed as pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid. For this problem, knowing the pKa of HF allows us to determine the required ratio of NaF to HF to achieve the target pH of 3.00.

Molar Mass and Mass Calculations

To determine how many grams of sodium fluoride (NaF) are needed, one must first calculate the number of moles required using the desired concentration and volume of the buffer solution. The molar mass of NaF is then used to convert moles into grams. This step is crucial for accurately preparing the buffer solution with the correct proportions of its components.

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Textbook Question

You are asked to prepare a pH = 3.00 buffer solution starting from 1.25 L of a 1.00 M solution of hydrofluoric acid (HF) and any amount you need of sodium fluoride (NaF). (a) What is the pH of the hydrofluoric acid solution prior to adding sodium fluoride?

Textbook Question

You are asked to prepare a pH = 4.00 buffer starting from 1.50 L of 0.0200 M solution of benzoic acid 1C6H5COOH2 and any amount you need of sodium benzoate 1C6H5COONa2. (a) What is the pH of the benzoic acid solution prior to adding sodium benzoate?

Textbook Question

You are asked to prepare a pH = 4.00 buffer starting from 1.50 L of 0.0200 M solution of benzoic acid 1C6H5COOH2 and any amount you need of sodium benzoate 1C6H5COONa2. (b) How many grams of sodium benzoate should be added to prepare the buffer? Neglect the small volume change that occurs when the sodium benzoate is added.

Textbook Question

A buffer contains 0.10 mol of acetic acid and 0.13 mol of sodium acetate in 1.00 L. (a) What is the pH of this buffer?