Question

How many grams of NaF would have to be added to 2.00 L of 0.100 M HF to yield a solution with a pH = 4.00? Ka for HF is 6.8 x 10^−4A) 300 gB) 36 gC) 60. gD) 6.9 g

Answer 1

To create a buffer solution with a pH of 4.00 using NaF and HF, you would need to add approximately 55 grams of NaF to a 2.00 L solution of 0.100 M HF, which is not one of the provided answer choices.

Step-by-step explanation:

To solve the problem, we will use the Henderson-Hasselbalch equation since it involves making a buffer solution with NaF and HF. Given a pH of 4.00 and a Ka for HF of 6.8 x 10⁻⁴, we need to find out how many grams of NaF are required to add to a 2.00 L solution of 0.100 M HF to achieve the desired pH.

To use the Henderson-Hasselbalch equation, which is pH = pKa + log([A-]/[HA]), first we need to calculate pKa as -log(6.8 x 10⁻⁴). With the pH and pKa, we can find the ratio of [A-] to [HA]. The desired ratio for a pH of 4.00 is 0.66 (base/acid).

For a 0.100 M HF solution (1 M for 2.00 L total volume), we require [F⁻] to equal 0.66 M. This equates to 0.66 moles per liter, and for 2.00 L of solution, we need 1.32 moles of F⁻. Since NaF dissociates completely in solution, the moles of NaF will also be 1.32. Multiplying by the molar mass of NaF (41.99 g/mol), this gives us 55.43 grams.

Therefore, none of the options A) 300 g, B) 36 g, C) 60 g, D) 6.9 g provided in the question is correct. The correct answer with the rounded figure would be: 55 g, which is not listed in the choices.

Answer 2

The number grams of NaF needed to yield a solution with pH 4.00 is D) 8.4 g of NaF.

Step-by-step explanation:

To calculate the grams of NaF needed, we first need to determine the concentration of F¯ that will result in a pH of 4.00. From the given Ka of HF, we can calculate the concentration of F¯ using the Henderson-Hasselbalch equation. The ratio of acid to conjugate base in a buffer solution is defined as follows:

pKa = pH - log([base]/[acid])

Since we know that the pH is 4.00, and the ratio should be 0.66, we can rearrange the equation:

[base]/[acid] = 10^(pKa - pH)

From the given Ka, we can calculate pKa:

pKa = -log(Ka) = -log(6.8 x 10^(-4)) = 3.17

Now we can substitute the values into the ratio equation:

[base]/[acid] = 10^(3.17 - 4.00) = 0.666

Next, we need to calculate the concentration of HF in the 2.00 L solution:

[HF] = 0.100 M x 2.00 L = 0.200 mol

Since HF and NaF have a 1:1 stoichiometric ratio, the concentration of F¯ will also be 0.200 M. Now we can calculate the grams of NaF needed:

0.200 M NaF x 41.99 g/mol NaF = 8.398 g

Therefore, the correct answer is D) 8.4 g of NaF.