Question

1. Using a 0.25 M phosphate buffer with a pH of 6.8, you add 0.79 mL...

1. Using a 0.25 M phosphate buffer with a pH of 6.8, you add 0.79 mL of 0.49 M HCl to 44 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)
________

2. Using a 0.25 M phosphate buffer with a pH of 6.8, you add 0.79 mL of 0.49 M NaOH to 44 mL of the buffer. What is the new pH of the solution? (Enter your answer to three significant figures.)
________

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Homework Answers

Answer #1

Moles of Phosphate buffer = 0.25 x 44 ml = 11 millimoles

After addition of HCl:

Moles of HCl added = 0.79 ml x 0.49 mol = 0.3871 millimoles

[Salt] = 11 - 0.3871 =10.6129 millimoles

[Acid] = 11 + 0.3871 = 11.3871 millimoles

pH = pKa + log ([Salt] / [Acid])

pH = 6.8 + log 10.6129/11.3871 = 6.8 - 0.0306 = 6.77

After addition of NaOH:

Moles of NaOH added = 0.79 ml x 0.49 mol = 0.3871 millimoles

[Salt] = 11 + 0.3871 = 11.3871 millimoles

[Acid] = 11 - 0.3871 = 10.6129 millimoles

pH = pKa + log ([Salt] / [Acid])

pH = 6.8 + log 11.3871/10.6129 = 6.8 + 0.0306 = 6.83

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