Question

In: Chemistry

A buffer solution consists of .195 M HCN and .22 M NaCN. a) what is the...

A buffer solution consists of .195 M HCN and .22 M NaCN.

a) what is the pH of this buffer solution?

b) you add 50 mL of .15 M HCl to 300.0 mL of this buffer solution. What will the pH be?

c) You add 20.0 mL of .26 M NaOH to 150.0 mL of this buffer solution. What will the pH be?

Solutions

Expert Solution

a) pH = pKa + log [CN^-]/[HCN]

pKa of HCN = 9.31

pH = 9.31 + log [0.22] / [0.195]

pH = 9.36

b) initially

millimoles of NaCN = 300 x 0.22 = 66.0

millimoles of HCN = 300 x 0.195 = 58.5

millimoles of HCl added = 50 x 0.15 = 7.50

after HCl added

millimoles of HCN = 58.5 + 7.50 = 66.0

millimoles of NaCN = 66.0 - 7.50 = 58.50

[NaCN] = 58.50 / 350 = 0.167 M

[HCN] = 66.0 / 350 = 0.188 M

pH = 9.31 + log [0.167] / [0.188]

pH = 9.26

c) millimoles of HCN = 150 x 0.195 = 29.25

millimoles of NaCN = 150 x 0.22 = 33.0

millimoles of NaOH = 20.0 x 0.26 = 5.20

after NaOH added

millimoles of HCN = 29.25 - 5.20 = 24.05

millimoles of NaCN = 33.0 + 5.20 = 38.20

[HCN] = 24.05 / 170 = 0.141 M

[NaCN] = 38.20 / 170 = 0.224 M

pH = 9.31 + log [0.224] / [0.141]

pH = 9.51

queen_honey_blossom answered 1 year ago

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