Question

Question 27 10 PES A student needed to make a buffer for her experiment with a pH = 9.25. She choose to use 21.00 mL of 0.270 M ammonia and 24.00 mL of 0.320 M ammonium chloride. The student mixed the two solutions together in a beaker and measured the pH. What is the expected pH of this buffer solution? What is the molarity of the ammonia once mixed in the buffer solution? [Select) What is the molarity of the sodium acetate once mixed in the buffer solution? (Select] What is the expected pH of this buffer solution? [Select] If the theoretical value of the buffer pH is 4.75, what is the percent error between his sample pH and the theoretical value? [Select] What could the student do in future experiments to make her buffer closer thing O fa Is 10 10 - # $ % & 3 6 7 8 9

Answer 1

Sol.

(a)

Millimoles of ammonia , NH3 = Molarity of NH3 × Volume of NH3  

= 0.270 M × 21 mL = 5.67 mmol

Millimoles of ammonium chloride , NH4Cl

= Molarity of NH4Cl × Volume of NH4Cl

= 0.320 M × 24 mL

= 7.68 mmol

Total Volume of solution = Volume of NH3 + Volume of NH4Cl

= 21 mL + 24 mL  

= 45 mL

Molarity of ammonia , NH3 once mixed in the buffer solution

= Millimoles of NH3 / Total Volume of solution

= 5.67 mmol / 45 mL

= 0.126 M

(b)

Molarity of ammonium chloride , NH4Cl once mixed in the buffer solution

= Millimoles of NH4Cl / Total Volume of solution

= 7.68 mmol / 45 mL

= 0.171 M

(c)

As pKa of NH4Cl is 9.25  

Therefore , Using Henderson - Hasselbalch equation ,

Expected pH = pKa + log ( Molarity of NH3 once mixed in the buffer solution / Molarity of NH4Cl once mixed in the buffer solution )

= 9.25 + log ( 0.126 M / 0.171 M )

= 9.11

(d)

Theoretical pH = 9.25  

Percent Error

= ( ( Theoretical pH - Expected pH ) / Theoretical pH ) × 100

= ( ( 9.25 - 9.11 ) / 9.25 ) × 100

= ( 0.14 / 9.25 ) × 100

= 1.5 %

(e)

She could use equimolar solutions of ammonia and ammonium chloride to make her buffer solution closer to the theoretical value