A buffer is prepared by mixing 20 mL of .10 HOCl with 20 mL of .05 M NaOCl, and measure the pH to be 7.15. Using this information calculate the Ka for HOCl
To calculate the equilibrium constant (Ka) for the dissociation of HOCl, we need to first determine the concentrations of the acid (HOCl) and its conjugate base (OCl-).
Given:
Volume of HOCl solution (V1) = 20 mL = 0.02 L
Volume of NaOCl solution (V2) = 20 mL = 0.02 L
Molarity of NaOCl solution = 0.05 M
We know that at equilibrium, the concentrations of HOCl and OCl- will be the same. Let's assume the final concentration of each is x.
Using the information provided, we know that the pH of the buffer solution is 7.15. Since HOCl is a weak acid, we can use the Henderson-Hasselbalch equation to relate the concentrations of HOCl and OCl- to the pH:
pH = pKa + log([OCl-]/[HOCl])
Since the pH is given as 7.15, we can substitute the values into the equation:
7.15 = pKa + log(x/x)
Simplifying, we get:
7.15 = pKa + log(1)
Since log(1) = 0, the equation simplifies further:
7.15 = pKa
Therefore, the pKa of HOCl is 7.15.
To calculate Ka, we can convert pKa to Ka using the equation: Ka = 10^(-pKa)
Therefore, Ka = 10^(-7.15)
Calculating this value gives us the answer.