Question

Calculate the pH after 0.012 mole of gaseous HCl is added to 243.0 mL of each of the following buffered solutions. (Assume that all solutions are at 25°C.) (a) 0.091 M C5H5N and 0.17 M C5H5NHCl 8.88 Incorrect: Your answer is incorrect. (b) 0.75 M C5H5N and 1.53 M C5H5NHCl

Answer 1

pH calculation

pKa of C5H5N = 5.25

(a) 0.091 M C5H5N and 0.17 M C5H5NHCl

pH of buffer by Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

addition of HCl would increase concentration of acid component of buffer and decrease concentration of base component of buffer

pH = 5.25 + log[(0.091 M x 0.243 L - 0.012 mol)/(0.17 M x 0.243 L + 0.012 mol)]

     = 4.53

(b)

(a) 0.75 M C5H5N and 1.53 M C5H5NHCl

pH of buffer by Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

addition of HCl would increase concentration of acid component of buffer and decrease concentration of base component of buffer

pH = 5.25 + log[(0.75 M x 0.243 L - 0.012 mol)/(1.53 M x 0.243 L + 0.012 mol)]

     = 4.897 (or rounded to 4.90 or 5.00)