You wish to prepare a buffer that can resist large additions of base, but not large dditions of acid. Which buffer system would be most appropriate? a) 0.2M HF/2.0M NaF b) 2.0M HF/2.0M NaF c) 2.0M HF/0.2M NaF d) 0.2M HF/0.2M NaF
Calculate the pH of a solution that is 0.25 M HF and 0.10 M NaF. Also, predict if these
concentrations are appropriate to make a buffer solution. (5 pts)
Ka = 3.5 x 10-4
HF ⇄ H
+
+ FHenderson-Hasselbalch Equation
?? = ??? + ???
[?
−]
[??]
?? = − log(3.4 ? 10
−4
) + ???
(0.10)
(0.25)
= 3.46 − 0.40 = ?. ??
Buffer Range
2.46 – 4.46
This solution is within the buffer range, thus the concentrations are appropriate to make a buffer
solution.
2. Calculate the pH of a buffer solution that is 0.20 M in HCN and 0.12 M in NaCN. Will the pH
change if the solution is diluted by a factor of 2? Explain to receive full credit. (5 pts)
Ka = 4.9 x 10-10
HCN ⇄ H
+ + CNHenderson-Hasselbalch Equation
?? = ??? + ???
[?
−]
[??]
?? = − log(4.9 ? 10
−10) + ???
(0.12)
(0.20)
= 9.31 − 0.22 = ?. ??
Dilution will not change the pH because the ratio of the conjugate base to acid will remain the
same.
[?
−]/2
[??]/2
=
[?
−]
[??]
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