If you make up a solution of 100 mL of 0.1 M HEPES in the basic form, what will be the pH?
I have no idea where to even start...
Just treat it as you would 0.1M soln of NH3.
For the sake of typing let's call the basic form of HEPES, BH2. In water solution this will ionize as
............BH2 + HOH ==> BH3^+ + OH^-
initial....0.1M.............0......0
change......-x.............x.......x
equil......0.1-x............x.......x
Kb = (BH3^+)(OH^-)/(BNH2)
Substitute from the ICE chart above, solve for OH^- and convert to pH.
To determine the pH of a solution, you need to consider the dissociation of the compound and its acid-base behavior. In this case, HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid) is a zwitterionic buffer, meaning it can exist in both acidic and basic forms.
To find the pH of a solution of 0.1 M HEPES (basic form), you need to know the pKa value of HEPES and its acid-base equilibrium constant (Ka). The pKa value will tell you at what pH the acidic and basic forms of HEPES are present in equal amounts. The formula to calculate pH is:
pH = pKa + log([A-]/[HA])
Where [A-] is the concentration of the basic form of HEPES and [HA] is the concentration of the acidic form (HA).
The pKa value of HEPES is approximately 7.55. Since the solution is 100 mL of 0.1 M HEPES in the basic form, the concentration of [A-] is 0.1 M.
Now, to calculate [HA], you need to consider that the concentration of [A-] and [HA] must add up to 0.1 M. So, [HA] = 0.1 M - [A-]. However, since the pKa value is close to the pH of the solution, [A-] will be much greater than [HA].
Therefore, you can approximate the concentration of [HA] to be negligible compared to [A-]. Hence, [HA] can be considered zero for practical purposes.
Using these values in the pH equation:
pH = pKa + log([A-]/[HA])
= 7.55 + log(0.1 M/0)
= 7.55 + log(infinity)
= 7.55 + infinity
The logarithm of infinity is undefined, so the pH of the solution is approximately equal to 7.55.
Note: This calculation assumes ideal conditions and neglects other factors that may affect the pH, such as ionic strength and temperature.