Question

3.Phosphate-buffered saline (PBS) is a buffer solution commonly used in biological research. Most formulations contain a mixture of KCl and NaCl buffered to a pH of 7.40 using phosphate-containing compounds.

a.The following reagents are available in your lab. Choose the correct reagents and calculate the volumes necessary to prepare exactly 5.0 L of a phosphate buffer with a pH of 7.40.

                                               0.100 M H3PO4 0.150 M Na2HPO4

                                               0.200 M KH2PO4 0.120 M Na3PO4

b.Commercial PBS solutions are typically prepared by dissolving solid reagents in water and then adjusting the pH to 7.40 using HCl. The formulation shown below is for the preparation of 10.0 L of a PBS solution. What is the pH of this solution and how many moles of HCl must be added to

adjust the pH to 7.40?

Component	Mass (g)
NaCl	80.0669
KCl	2.0129
Na2HPO4	14.1960
KH2PO4	2.4496

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Question

1. When NO2 is bubbled into water, it is completely converted to HNO3 and HNO2 according to the reaction shown below. Calculate the pH and the molar concentrations of all dissolved species (H3O+, OH–, HNO2, HNO3, NO2–, and NO3–) in a solution prepared by bubbling 4.601 g NO2 through 2.00 L of water.

2NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)

2. 12.6-g sample of Na2A (molar mass = 126 g/mol) is dissolved enough water to produce 1.00 L of solution. The pH of this solution is 9.85 and the equilibrium concentration of H2A is 1.0 × 10–12 M. Determine the values of pKa1 and pKa2 for H2A

3.Phosphate-buffered saline (PBS) is a buffer solution commonly used in biological research. Most formulations contain a mixture of KCl and NaCl buffered to a pH of 7.40 using phosphate-containing compounds.

a.The following reagents are available in your lab. Choose the correct reagents and calculate the volumes necessary to prepare exactly 5.0 L of a phosphate buffer with a pH of 7.40.

                                               0.100 M H3PO4 0.150 M Na2HPO4

                                               0.200 M KH2PO4 0.120 M Na3PO4

b.Commercial PBS solutions are typically prepared by dissolving solid reagents in water and then adjusting the pH to 7.40 using HCl. The formulation shown below is for the preparation of 10.0 L of a PBS solution. What is the pH of this solution and how many moles of HCl must be added to

adjust the pH to 7.40?

Component	Mass (g)
NaCl	80.0669
KCl	2.0129
Na2HPO4	14.1960
KH2PO4	2.4496

4. Consider the potentiometric titration of 25.0 mL of 0.200 M selenious acid (H2SeO3) with 0.250 M NaOH.

a. Identify the primary selenium-containing species present at each point in the titration (A – E) shown below. Then sketch the titration curve, identifying points A – E. Make sure to appropriately label the axes.

Point	NaOH Added (mL)		Se-Containing Species
A		0.00
B		10.00
C		20.00
D		25.00
E		40.00

Answer 1

3.

a). We know that,

Ka of H₃PO₄ = 6.2X10^-8

pKa = - logKa

= - log(6.2X10^-8 )

= 7.20

pH of buffer = 7.4

pH = pKa + log[salt] / [acid]

7.40 = 7.20 + log[salt] / [acid]

log[salt] / [acid] = 0.20

[salt] / [acid] = 10^0.20

[salt] / [acid] = 1.584

For 0.100 M H₃PO₄ 0.150 M Na₂HPO₄

[Na₂HPO₄] / [H₃PO₄] = 0.150 / 0.100

= 1.5

For 0.200 M KH₂PO₄ 0.120 M Na₃PO₄

[Na₃PO₄] / [KH₂PO₄] = 0.120 / 0.200

= 0.6 1.58

So, 0.200 M KH₂PO₄ 0.120 M Na₃PO₄ are necessary to prepare exactly 5.0 L of a phosphate buffer with a pH of 7.40.

1. 2NO₂(g) + H₂O(l) → HNO₃(aq) + HNO2(aq)

mass of NO2 = 4.601 gm

moles of NO2 = 4.601 / 46 = 0.1

2 moles of No2 give - 1 mole of HNO3 and 1 mole of HNO2

0.1 moles of NO2 will give = 0.05 mole of HNO3 and 0.05 mole of HNO2

volume of solution = 2L

[HNO3] = 0.05 / 2

[HNO3] = 0.025 M

[HNO2] = 0.025 M

Now, as HNO3 is a strong acid it completely dissociates to give H+ and NO3- ions.

HNO3 H+ + NO3-

[H+] = [HNO3] = 0.025 M

[NO3-] = [HNO3] = 0.025 M

Similarly,

HNO2 H+ + NO2-

[H+] = 0.025 M

[NO2-] = 0.025 M

We know that,

[H+][OH-] = 10^-14

0.025[OH-] = 10^-14

[OH-] = 4X10^-3 M

pH = - log[H+]

= - log(0.025)

   = 1.601