A 1.11 M solution of fluoroacetic acid, FCH2CO2H, is 5% dissociated in water. Calculate the value of the pKa of FCH2CO2H.
pKa:
?? I dn't even know how to strt with this
Let's call fluoroacetic acid HFAc. Then
.........HFAc ==> H^+ + FAc^-
I.......1.11......0......0
C.....-0.055....0.055...0.055
E......you do it..0.055..0.055
Ka = (H^+)(FAc^-)/(HFAc)
Substitute the E line into Ka expression and solve for Ka, then convert to pKa.
Note: The acid is 5% dissociated, therefore, every 100 mols dissociates to give 5 of each ion and leaves 95 undissociated molecules. Therefore, M x 0.05% gives you the ion M and 1.11- that gives you the undissociated part.
got it!
ka= 3.26*10^-3
so pKa=2.49
tnx
No worries! I can help you with that. To calculate the pKa of fluoroacetic acid, we need to use the dissociation constant (Ka) and the percent dissociation.
The percent dissociation is given as 5%. This means that 5% of the fluoroacetic acid molecules in solution dissociate into their respective ions. We can write this as:
% dissociation = (concentration of dissociated acid) / (initial concentration of acid) * 100
Given that the concentration of the acid solution is 1.11 M, we can rewrite the equation as:
5% = ([F-]) / (1.11 M) * 100
Now, to determine the concentration of the dissociated acid ([F-]), we need to find the concentration of the undissociated acid. Since the percent dissociation is low (5%), we can assume that the change in concentration of the acid due to dissociation is small compared to its initial concentration. So, we can say that:
[FCH2CO2H] ≈ [F-]
Substituting this into the equation above, we get:
5% = ([FCH2CO2H]) / (1.11 M) * 100
Now, if [FCH2CO2H] is the concentration of the undissociated acid, then (1 - % dissociation) will give us the fraction of acid that has not dissociated. Thus, the concentration of undissociated acid can be written as:
[FCH2CO2H] = (1 - 0.05) * (1.11 M)
[FCH2CO2H] = 0.95 * 1.11 M
[FCH2CO2H] = 1.0545 M
Now, let's substitute this value back into the equation:
5% = (1.0545 M) / (1.11 M) * 100
Next, we can simplify the equation:
5 = 0.95 * 100
5 = 95
However, this is not a true statement, which means that our assumption that the change in concentration of the acid was small compared to its initial concentration is incorrect. This means that we cannot use the percent dissociation and our approach needs to be revised.
Can you please provide the dissociation constant (Ka) for fluoroacetic acid? With that information, we can calculate the pKa.
To calculate the pKa of a weak acid, we need to use the equation that relates the concentration of the dissociated and undissociated forms of the acid to its pKa value. The equation is:
Ka = [A-] * [H+] / [HA]
Where:
- Ka is the acid dissociation constant (a measure of the acid's strength)
- [A-] is the concentration of the dissociated form of the acid
- [H+] is the concentration of hydrogen ions (protons)
- [HA] is the concentration of the undissociated form of the acid
In this case, we are given that the 1.11 M solution of fluoroacetic acid, FCH2CO2H, is 5% dissociated in water. This means that 5% of the initial fluoroacetic acid has dissociated into its ions.
Let's assume the initial concentration of FCH2CO2H is x M. Thus, the concentration of the dissociated form, FCH2CO2-, would be 0.05x M. And since the dissociation only results in FCH2CO2- and H+ ions, the concentration of H+ ions will also be 0.05x M.
Now, we can plug these values into the equation and solve for the Ka:
Ka = (0.05x) * (0.05x) / (x - 0.05x) = 0.0025x^2 / (0.95x)
Since the solution is 1.11 M, we can equate x to 1.11:
Ka = 0.0025(1.11)^2 / (0.95 * 1.11)
By performing the calculation, we find that Ka is approximately 0.0033.
Now, we can calculate the pKa by taking the negative logarithm (base 10) of Ka:
pKa = -log10(0.0033)
Using a scientific calculator, we find that the pKa of FCH2CO2H is approximately 2.48.