CHEM:BUFFERS DYNAMIC STUDY MODULE

What mass of NaCN must be added to create 250.0 mL of a buffer solution with a pH = 9.00 and an HCN concentration of 0.100 M? The pKa of hydrocyanic acid is 9.23.

0.72 g 9.00 = 9.23 + log(x/0.100) -0.23 = log(X/0.100) 10^-0.23 = X/0.100 X= 0.059 250.0 mL * 0.059 mol/ 1L * 1 mol/ 1 mol * 49.01 g/ 1 mol = 0.79 g NaCN

Which pair of solutions will form an effective buffer?

1.0 M HCN and 0.30 M KCN

What is the pH of a buffer system that has 0.35 M CH3NH2 and 0.42 M CH3NH3+? The Kb of methylamine is 4.4 × 10−4.

10.56 pKb = −log Kb pKb = −log (4.4 × 10−4) pKb = 3.36 pOH= 3.36 + log(0.42 M/ 0.35 M) pOH= 3.36 + 0.08 pOH= 3.44 pH + pOH = 14 14- 3.44 pH= 10.56

What mass of CH3COONa must be added to create 300.0 mL of a buffer solution with a pH = 5.00 and an CH3COOH concentration of 0.100 M? The pKa of acetic acid is 4.75.

4.4 g 5.00 = 4.75 + log(X/0.100) 0.25 = log(X/0.100) 10^0.25 = X/0.100 X= 0.18 300.0 mL * 1L/1,000mL * 0.18 mol/ 1L * 1 mol/1 mol * 82.03 g/ 1 mol = 4.4 g CH3COONa

A buffer solution has 0.625 M HCN and 0.575 M CN−. If 0.030 moles of HCl is added to 400. mL of the buffer solution, what is the pH after the addition? The pKa of hydrocyanic acid is 9.23.

9.08 400. mL buffer * 1L/ 1,000 mL * 0.625 mol HCN/ 1L solution = 0.250 mol HCN 400. mL buffer * 1L/1,000 mL* 0.575 mol CN-/ 1L solution = 0.230 mol CN- pH= 9.23 + log(0.230 mol - 0.030 mol/ 0.250 mol + 0.030 mol) pH= 9.23-0.146 pH= 9.08

What is the pH of a buffer system that has 0.32 M NH4+ and 0.27 M NH3? The pKa of ammonium ion is 9.25.

9.18 pH= 9.25 + log(0.27/0.32) pH= 9.25 -0.07 pH= 9.18

What is the pH of a buffer system that has 0.58 M NH3 and 0.41 M NH4+? The Kb of ammonia is 1.8 × 10−5.

9.41 pOH= pKb + log([NH4+/NH3]) pOH= 4.74 + log(0.41 M/ 0.58M) pOH= 4.74 - 0.15 pOH= 4.59 pH + pOH= 14 14- 4.59 pH= 9.41

Select the best buffer to maintain a solution at pH = 3.5

HCOOH with HCOONa, Ka of HCOOH = 1.8 x 10−4

Select the best buffer to maintain a solution at pH = 8.0.

HClO with NaClO, Ka of HClO = 3.5 × 10−8

Which pair of aqueous solutions can create a buffer solution if present in the appropriate concentrations?

HF and NaF

Which pair of aqueous solutions can create a buffer solution if present in the appropriate concentrations?

NH3 and NH4Cl

Which pair of aqueous solutions can create a buffer solution if present in the appropriate concentrations?

NaH2PO4 and Na2HPO4

A solution is buffered at a pH of 3.5 and a small amount of acid is added. What happens to the pH of the solution?

The pH will decrease a small amount.

A solution is buffered at a pH of 6.5 and a small amount of base is added. What happens to the pH of the solution?

The pH will increase a small amount.

Which pair of solutions will form an effective buffer?

0.40 M HF and 0.30 M NaF

Which pair of solutions will form an effective buffer?

0.75 M H3PO4 and 0.45 M NaH2PO4

What mass of KNO2 must be added to create 200.0 mL of a buffer solution with a pH = 3.00 and an HNO2 concentration of 0.150 M? The pKa of nitrous acid is 3.34.

1.2 g 3.00 = 3.34 + log(X/0.150 M) -0.34 = log(X/0.150 M) 10^-0.34 = X/ 0.150 X= 0.069 M 200.0 mL * 1L/1,000 mL * 0.069 mol/ 1 L * 1 mol/ 1 mol * 85.10 g / 1 mol = 1.2 g KNO2

What is the pH of a buffer system that has 0.87 M (CH3)2NH and 0.68 M (CH3)2NH2+? The Kb of methylamine is 5.9 × 10−4.

10.88 pKb = −log Kb pKb = −log (5.9 × 10−4) pKb = 3.23 pOH= 3.23 + log(0.60M / 0.87M) pOH= 3.23 - 0.11 pOH= 3.12 pH + pOH = 14 14- 3.12 pH= 10.88

What mass of NaF must be added to create 250.0 mL of a buffer solution with a pH = 3.45 and an HF concentration of 0.100 M? The pKa of hydrofluoric acid is 3.16.

2.0 g 3.45 = 3.16 + log(X/0.100 M) 0.29 = log(X/0.100 M) 10^0.29 = X/0.100 M X= 0.19 M 250.0 mL * 1L/1,000 mL * 0.19 mol F-/1L * 1 mol NaF/ 1 mol F- * 41.99g/ 1 mol NaF = 2.0 g

A buffer solution has 0.620 M HF and 0.710 M F−. If 0.025 moles of HCl is added to 325 mL of the buffer solution, what is the pH after the addition? The pKa of hydrofluoric acid is 3.16.

3.12 325 mL buffer * 1L/ 1,000 mL * 0.620 mol HF/ 1L solution = 0.202 mol HF 325 mL buffer* 1L/1,000 mL * 0.710 mol F-/ 1L solution = 0.231 mol F- pH= 3.16 + log (0.231 mol - 0.025 mol / 0.202 mol + 0.025 mol) pH= 3.16 - 0.042 pH= 3.12

A buffer solution has 0.620 M HF and 0.710 M F−. If 0.025 moles of NaOH is added to 325 mL of the buffer solution, what is the pH after the addition? The pKa of hydrofluoric acid is 3.16.

3.32 325 mL buffer * 1L/1,000 mL * 0.620 mol HF/ 1L solution= 0.202 mol HF 325 mL buffer * 1L/1,000 mL * 0.710 mol F-/ 1L solution = 0.231 mol F- pH= 3.16 + log(0.231 mol + 0.025 mol/ 0.202 mol - 0.025) pH= 3.16 + 0.160 pH= 3.32

What is the pH of a buffer system that has 0.11 M CH3COOH and 0.15 M CH3COO−? The pKa of acetic acid is 4.75.

4.88 pH = 4.75 + log(0.15/0.11) pH= 4.75 + 0.13 pH = 4.88

A buffer solution has 0.750 M H2CO3 and 0.650 M HCO3−. If 0.020 moles of HCl is added to 275 mL of the buffer solution, what is the pH after the addition? The pKa of carbonic acid is 6.37.

6.22 275 mL buffer * 1L/1,000 mL * 0.750 mol/ 1L solution = 0.206 mol H2CO3 275 mL buffer * 1L/1,000 mL * 0.650 mol/ 1L solution = 0.179 mol HCO3- pH= 6.37 + log(0.179 mol - 0.020 mol/ 0.206 mol + 0.020 mol) pH= 6.37 - 0.153 pH= 6.22

A buffer solution has 0.750 M H2CO3 and 0.650 M HCO3−. If 0.020 moles of NaOH is added to 275 mL of the buffer solution, what is the pH after the addition? The pKa of carbonic acid is 6.37.

6.40 why? 275mL buffer * 1L/1,1000 mL * 0.75 mol H2CO3/ 1L Solution = 0.206 mol H2CO3 275 mL buffer * 1L/ 1,000 mL * 0.65 mol HCO3- / 1L Solution= 0.179 mol HCO3- pH= 6.37 + log(0.179 mol + 0.020 mol / 0.206 mol + 0.020 mol) pH= 6.37 + 0.0293 pH+ 6.40

What is the pH of a buffer system that has 0.15 M H2CO3 and 0.20 M HCO3−? The pKa of carbonic acid is 6.35.

6.49

A buffer solution has 0.625 M HCN and 0.575 M CN−. If 0.030 moles of NaOH is added to 400. mL of the buffer solution, what is the pH after the addition? The pKa of hydrocyanic acid is 9.23.

9.30 400. mL buffer * 1L/ 1,000 mL * 0.625 mol HCN/ 1L solution = 0.250 mol HCN 400.mL buffer * 1L/1,000 mL * 0.575 mol CN-/ 1L solution = 0.230 mol CN- pH= 9.32 + log(0.230 mol + 0.030 mol/ 0.250 mol - 0.030 mol) pH= 9.32 + 0.073 pH = 9.30

What is the pH of a buffer system that has 0.25 M HCN and 0.38 M CN−? The pKa of hydrocyanic acid is 9.23.

9.49 pH= 9.23 + log(0.38M / 0.25 M) pH= 9.23 + 0.18 pH= 9.41

Select the best buffer to maintain a solution at pH = 9.0.

HBrO with NaBrO, Ka of HBrO = 2.0 × 10−9

Select the best buffer to maintain a solution at pH = 2.9.

HF with NaF, Ka of HF = 7.2 × 10−4