Using a 025 M phosphate buffer wi 73 you add 075 mL of 055 M

Using a 0.25 M phosphate buffer wi 7.3, you add 0.75 mL of 0.55 M HC to 45 mL of the buffer (Enter your answer to three significant figures.) : What is the new pH of the solution? Using a 0.25 M phosphate buffer with a pH of 7.3, you add 0.75 mL of 0.55 M NaOH to 45 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.) Supporting Materials

Solution

pH = 7.3

pKa = 7.21

pH = pKa + log [salt / acid]

7.3 = 7.21 + log [salt / acid]

[salt / acid] = 1.23

[salt + acid] = 0.25

1.23 acid + acid = 0.25

concentration of acid = 0.1121

concentration of salt = 0.1379

mmoles of acid = 0.1121 x 45 = 5.044

mmoles of salt = 0.1379 x 45 = 6.204

a)

mmoles of HCl added = 0.75 x 0.55 = 0.4125

pH = pKa + log [salt - C / acid + C]

     = 7.21 + log [6.204 - 0.4125 / 5.044 + 0.4125]

pH = 7.24

b)

mmoles of NaOH added = 0.75 x 0.55 = 0.4125

pH = pKa + log [salt + C / acid - C]

     = 7.21 + log [6.204 + 0.4125 / 5.044 - 0.4125]

pH = 7.36