EXAMPLE PROBLEM APPLYING THE HENDERSON-HASSELBALCH EQUATION Calculate the pH of

Question

EXAMPLE PROBLEM APPLYING THE HENDERSON-HASSELBALCH EQUATION Calculate the pH of a buffer solution made from 0.20 M HC_2H_3O_2 and 0.50 M C_2H_3O_2^- that has an acid dissociation constant for HC_2H_3O_2 of 1.8 times 10^-5. Solve this problem by plugging the values into the Henderson-Hasselbalch equation for a weak acid and its conjugate base. pH = pK_a + log ([A^-]/[HA]) pH = pK_a + log ([C_2H_3O^-_2]/[HC_2H_3O_2]) pH = -log (1.8 times 10^-5) + log (0.50 M/0.20 M) pH = -log (1.8 times 10^-5) + log (2.5) pH = 4.7 + 0.40 pH =5.1

Explanation / Answer

Ka = 1.8*10^-5

PKa = -log1.8*10^-5

         = 4.74

PH   = Pka + log[A-]/[HA]

HA is Acid = HC2H3O2 is aacid

A- is conjugate base = C2H3O2- is conjugate base

So [HA] = 0.2M

    [A^-] = 0.5M

PH   = Pka + log[A-]/[HA]

       = 4.74 + log0.5/0.2

       = 4.74 + 0.3979   = 5.1379

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