Find the grams of NaF added to 300ml of .2M HF to create a buffer with pH=3.5.
Use the Henderson-Hasselbalch equation.
pH = pKa + log [(base)/(acid)]
what is the Ka of HF?
I have a quant book that lists it as Ka = 7.2 x 10^-4
I came up with -5.4 x 10^-4.
What's with the negative sign? Is that a value for Ka you found different from mine. The negative value makes no sense. Or is that the grams NaF to add? Again, the negative sign makes no sense.
I don't know what happened to my response to your post but I made it last night. It showed up on the boards but today it doesn't show UNLESS you click on the main link (the first link) to the question, then all the answers show. At any rate, click on that to get the response. Bascially, I don't understand the negative sign.
To find the grams of NaF added to create a buffer with pH=3.5, we need to first calculate the concentration of the buffer solution.
A buffer solution is made up of a weak acid and its conjugate base, which in this case is HF and its conjugate base, NaF. The Henderson-Hasselbalch equation allows us to relate pH, pKa, and the concentrations of the acid and conjugate base:
pH = pKa + log([conjugate base]/[weak acid])
We are given that the pH of the buffer solution is 3.5, and we know that HF is a weak acid. The pKa of HF is the negative logarithm of its acid dissociation constant (Ka).
To calculate the concentration of the buffer, we will assume that the dissociation of HF is negligible. Therefore, the concentration of the weak acid HF can be considered to be the same as the initial concentration of the solution, which is 0.2M.
Next, we need to find the concentration of the conjugate base NaF. Since we are given the volume of the solution (300ml), we can find the number of moles of NaF using the concentration equation:
moles = concentration × volume (in liters)
moles of NaF = (0.2M) × (0.3L) = 0.06 moles
Finally, we can find the grams of NaF using the molar mass of NaF. The molar mass of NaF is approximately 41 grams per mole.
grams = moles × molar mass
grams of NaF = (0.06 moles) × (41 g/mol) ≈ 2.46 grams
Therefore, approximately 2.46 grams of NaF should be added to 300ml of 0.2M HF to create a buffer with a pH of 3.5.