Find the grams of NaF added to 300ml of .2M HF to create a buffer with pH=3.5.
Answered above.
To find the grams of NaF required to create a buffer with pH 3.5, we need to determine the amount of HF (hydrofluoric acid) and NaF (sodium fluoride) needed to achieve the specific pH.
First, let's understand the concept of buffers and the Henderson-Hasselbalch equation. A buffer solution consists of a weak acid and its conjugate base (or a weak base and its conjugate acid). The buffer helps maintain the pH of a solution by resisting changes in acidity or alkalinity.
The Henderson-Hasselbalch equation is used to calculate the pH of a buffer solution and is defined as:
pH = pKa + log([A-]/[HA])
Here, pH represents the desired pH of the buffer, pKa is the acid dissociation constant of HF, and [A-] and [HA] are the concentrations of the conjugate base (F-) and weak acid (HF), respectively.
Given that the desired pH is 3.5, we can calculate the pKa using the equation pKa = -log(Ka), where Ka is the acid dissociation constant of HF. It is approximately 7.2 × 10^-4 for HF.
pKa = -log(7.2 × 10^-4) ≈ 3.14
Using the Henderson-Hasselbalch equation, we can rearrange it to solve for the ratio [A-]/[HA]:
[A-]/[HA] = 10^(pH - pKa)
[A-]/[HA] = 10^(3.5 - 3.14) ≈ 1.76
Since the ratio of [A-] to [HA] is 1.76, we know that for each mole of HF, we need 1.76 moles of F-. However, we are given the molarity of HF and the volume of the solution, so we need to convert these values to moles.
Molarity (M) = moles/volume (in liters)
For HF:
0.2 M = moles_HF/0.3 L
moles_HF = 0.2 M * 0.3 L = 0.06 moles
Since the stoichiometry ratio tells us that we need 1.76 moles of F- per 1 mole of HF, we can calculate the moles of F- required:
moles_F- = 0.06 moles_HF * 1.76 = 0.1056 moles
The last step is to convert moles of NaF to grams:
Grams = moles_F- * molar mass of NaF
The molar mass of NaF is approximately 41 grams/mol. Therefore:
Grams = 0.1056 moles * 41 grams/mol ≈ 4.33 grams
Therefore, you would need approximately 4.33 grams of NaF to add to 300 mL of 0.2 M HF to create a buffer with a pH of 3.5.