Question

Topic: pH and K_a Value

Note:

y-intercept = 8.46

Henderson-Hasselbalch equation : pH = pKa + log[A^-/HA]

Literature value of pK_a = 9.245

Question: Use Henderson-Hasselbalch equation and y-intercept to determine the K_a value for the acid in the conjugate pair (NH₃ and NH₄Cl). Show your work.

Answer 1

Answer – We are given the Henderson-Hasselbalch equation and graph between the pH vrs log [NH₃]/[NH₄Cl]

We know the Henderson-Hasselbalch equation-

pH = pKa + log[A^-/HA]

From the graph , y-intercept = 8.46

When we draw the graph between the pH vrs log salt/acid then there is straight line equation and the intercept is the pKa values.

So,    y-intercept = pKa = 8.46 when graph between the pH vrs log salt/acid.

We know,

pKa = - log Ka

so, Ka = 10^-pKa

           = 10^-8.46

           = 3.46*10^-9

The actual Ka from pKa = 9.245 is 5.68*10^-10, so there is maybe some error in the calculation or recording data.