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General Chemistry

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18. Aqueous Equilibrium

Buffer

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PRACTICE 124

Textbook Question

Consider the following table of standard electrode potentials for a series of hypothetical reactions in an aqueous solution: reduction half-reaction E °(V) (c) Which substance(s) can oxidize C2+?

Textbook Question

A mixture of copper and gold metals that is subjected to electrorefining contains tellurium as an impurity. The standard reduction potential between tellurium and its lowest common oxidation state, Te4+, is Te4+1aq2 + 4 e- ¡ Te1s2 E°red = 0.57 V Given this information, describe the probable fate of tellurium impurities during electrorefining. Do the impurities fall to the bottom of the refining bath, unchanged, as copper is oxidized, or do they go into solution as ions? If they go into solution, do they plate out on the cathode?

Textbook Question

(b) What is the most significant difference between the sulfides precipitated in group 2 and those precipitated in group 3?

Textbook Question

A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2 +1aq2 and 0.010 M in Sr2 +1aq2. (c) What is the concentration of SO42 - 1aq2 when the second cation begins to precipitate?

Textbook Question

The value of Ksp for Cd1OH22 is 2.5 * 10-14. (b) The solubility of Cd1OH22 can be increased through formation of the complex ion CdBr42 - 1Kf = 5 * 1032. If solid Cd1OH22 is added to a NaBr solution, what is the initial concentration of NaBr needed to increase the molar solubility of Cd1OH22 to 1.0 * 10-3 mol/L?

Textbook Question

The solubility product for Zn1OH22 is 3.0 * 10-16. The formation constant for the hydroxo complex, Zn1OH242 - , is 4.6 * 1017. What concentration of OH- is required to dissolve 0.015 mol of Zn1OH22 in a liter of solution?

Textbook Question

From the value of Kf listed in Table 17.1, calculate the concentration of Ni2 +1aq2 and Ni1NH326 2+ that are present at equilibrium after dissolving 1.25 g NiCl2 in 100.0 mL of 0.20 M NH31aq2.

Textbook Question

(b) As the acid is titrated, the pH of the solution after the addition of 11.05 mL of the base is 4.89. What is the Ka for the acid?

Textbook Question

Assume that 30.0 mL of a 0.10 M solution of a weak base B that accepts one proton is titrated with a 0.10 M solution of the monoprotic strong acid HA. (b) What is the predominant form of B at the equivalence point?

Textbook Question

Which of the following solutions is a buffer? (a) A solution made by mixing 100 mL of 0.100 M CH3COOH and 50 mL of 0.100 M NaOH, (b) a solution made by mixing 100 mL of 0.100 M CH3COOH and 500 mL of 0.100 M NaOH, (c) A solution made by mixing 100 mL of 0.100 M CH3COOH and 50 mL of 0.100 M HCl, (d) A solution made by mixing 100 mL of 0.100 M CH3COOK and 50 mL of 0.100 M KCl.

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (c) Which of the three represents a situation that cannot arise from the addition of either an acid or a base? [Section 17.2]

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (a) Which of the three drawings, (1), (2), or (3), represents the buffer after the addition of a strong acid? [Section 17.2]

Textbook Question

The beaker on the right contains 0.1 M acetic acid solution with methyl orange as an indicator. The beaker on the left contains a mixture of 0.1 M acetic acid and 0.1 M sodium acetate with methyl orange. (b) Which solution is better able to maintain its pH when small amounts of NaOH are added? Explain. [Sections 17.1 and 17.2]

Textbook Question

Mathematically prove that the pH at the halfway point of a titration of a weak acid with a strong base (where the volume of added base is half of that needed to reach the equivalence point) is equal to pKa for the acid.

Textbook Question

Two buffers are prepared by adding an equal number of moles of formic acid (HCOOH) and sodium formate (HCOONa) to enough water to make 1.00 L of solution. Buffer A is prepared using 1.00 mol each of formic acid and sodium formate. Buffer B is prepared by using 0.010 mol of each. (b) Which buffer will have the greater buffer capacity?

Textbook Question

Derive an equation similar to the Henderson–Hasselbalch equation relating the pOH of a buffer to the pKb of its base component.

Textbook Question

You have to prepare a pH = 3.50 buffer, and you have the following 0.10 M solutions available: HCOOH, CH3COOH, H3PO4, HCOONa, CH3COONa, and NaH2PO4. Which solutions would you use?

Textbook Question

A buffer contains a weak acid, HA, and its conjugate base. The weak acid has a pKa of 4.5, and the buffer has a pH of 4.3. Without doing a calculation, state which of these possibilities are correct at pH 4.3. (a) 3HA4 = 3A-4, (b) 3HA4 7 3A-4, or (c) 3HA4 6 3A-4. [Section 17.2]

Textbook Question

The acid–base indicator bromcresol green is a weak acid. The yellow acid and blue base forms of the indicator are present in equal concentrations in a solution when the pH is 4.68. What is the pKa for bromcresol green?

Textbook Question

The beaker on the right contains 0.1 M acetic acid solution with methyl orange as an indicator. The beaker on the left contains a mixture of 0.1 M acetic acid and 0.1 M sodium acetate with methyl orange. (a) Using Figures 16.8 and 16.9, which solution has a higher pH?

Textbook Question

The indicator methyl orange has been added to both of the following solutions. Based on the colors, classify each statement as true or false: (a) The pH of solution A is definitely less than 7.00.

Textbook Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (c) What other indicator or indicators would you want to use to determine the pH of the solution more precisely?

Textbook Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (b) Which of the following can you establish about the solution: (i) A minimum pH, (ii) A maximum pH, or (iii) A specific range of pH values?

Textbook Question

Addition of the indicator methyl orange to an unknown solution leads to a yellow color. The addition of bromthymol blue to the same solution also leads to a yellow color. (b) What is the range (in whole numbers) of possible pH values for the solution?

Textbook Question

Is each of the following substances likely to serve as an oxidant or a reductant: (d) N2O51g2?

Textbook Question

By using the data in Appendix E, determine whether each of the following substances is likely to serve as an oxidant or a reductant: (d) Zn(s)

Textbook Question

By using the data in Appendix E, determine whether each of the following substances is likely to serve as an oxidant or a reductant: (b) MnO4- (aq, acidic solution),

Textbook Question

Which metal cation is the best oxidizing agent? a. Pb2+ b. Cr3+ c. Fe2+ d. Sn2+

Textbook Question

Which metal can be oxidized with an Sn2+ solution but not with an Fe2+ solution?

Textbook Question

A 25.0-mL sample of 0.125 M pyridine is titrated with 0.100 M HCl. Calculate the pH at each volume of added acid: one-half equivalence.

Textbook Question

A 30.0-mL sample of 0.165 M propanoic acid is titrated with 0.300 M KOH. Calculate the pH at each volume of added base: one-half equivalence point.

Textbook Question

A solution is 0.010 M in Ba2 + and 0.020 M in Ca2 + . b. What is the remaining concentration of the cation that precipitates first, when the other cation begins to precipitate?

Textbook Question

A solution is 0.010 M in Ba2 + and 0.020 M in Ca2 + . a. If sodium sulfate is used to selectively precipitate one of the cations while leaving the other cation in solution, which cation will precipitate first? What minimum concentration of Na2SO4 will trigger the precipitation of the cation that precipitates first?

Textbook Question

Calculate the solubility of CuX in a solution that is 0.150 M in NaCN. Ksp for CuX is 1.27 * 10 - 36.

Textbook Question

Use the appropriate values of Ksp and Kf to find the equilibrium constant for the reaction. FeS(s) + 6 CN-(aq)ΔFe(CN)64 - (aq) + S2 - (aq)

Textbook Question

A solution is made 1.1 * 10 - 3 M in Zn(NO3)2 and 0.150 M in NH3. After the solution reaches equilibrium, what concentration of Zn2 + (aq) remains?

Textbook Question

The fluids within cells are buffered by H2PO4- and HPO42 - . b. Could a buffer system employing H3PO4 as the weak acid and H2PO4- as the weak base be used as a buffer system within cells? Explain.

Textbook Question

Determine whether or not the mixing of each pair of solutions results in a buffer. b. 150.0 mL of 0.10 M HF; 135.0 mL of 0.175 M HCl

Textbook Question

A buffer contains significant amounts of ammonia and ammonium chloride. Write equations showing how this buffer neutralizes added acid and added base.

Textbook Question

A 500.0-mL buffer solution is 0.100 M in HNO2 and 0.150 M in KNO2. Determine if each addition would exceed the capacity of the buffer to neutralize it. c. 1.25 g HBr

Textbook Question

A 500.0-mL buffer solution is 0.100 M in HNO2 and 0.150 M in KNO2. Determine if each addition would exceed the capacity of the buffer to neutralize it. a. 250 mg NaOH

Textbook Question

What should the molar concentrations of benzoic acid and sodium benzoate be in a solution that is buffered at a pH of 4.55 and has a freezing point of -2.0 °C? (Assume complete dissociation of sodium benzoate and a density of 1.01 g/mL for the solution.)

Textbook Question

A 1.0-L buffer solution initially contains 0.25 mol of NH3 and 0.25 mol of NH4Cl. In order to adjust the buffer pH to 8.75, should you add NaOH or HCl to the buffer mixture? What mass of the correct reagent should you add?

Textbook Question

What mass of sodium benzoate should you add to 150.0 mL of a 0.15 M benzoic acid solution to obtain a buffer with a pH of 4.25? (Assume no volume change.)

Textbook Question

Use the Henderson–Hasselbalch equation to calculate the pH of each solution. c. a solution that contains 10.0 g of HC2H3O2 and 10.0 g of NaC2H3O2 in 150.0 mL of solution

Textbook Question

Referring to Table 17.1, pick an indicator for use in the titration of each base with a strong acid. a. CH3NH2

Textbook Question

Referring to Table 17.1, pick an indicator for use in the titration of each acid with a strong base. a. HF

Textbook Question

Methyl red has a pKa of 5.0 and is red in its acid form and yellow in its basic form. If several drops of this indicator are placed in a 25.0-mL sample of 0.100 M HCl, what color will the solution appear? If 0.100 M NaOH is slowly added to the HCl sample, in what pH range will the indicator change color?

Textbook Question

Consider the curve shown here for the titration of a weak base with a strong acid and answer each question.

c. At what volume of added acid does pH = 14 - pKb?

Textbook Question

Consider the curve shown here for the titration of a weak monoprotic acid with a strong base and answer each question.

c. At what volume of added base does pH = pKa?

Textbook Question

You are titrating an acidic solution with a basic one, and just realized you forgot to add the indicator that tells you when the equivalence point is reached. In this titration, the indicator turns blue at the equivalence point from an initially colorless solution. You quickly grab a bottle of indicator and add some to your titration beaker, and the whole solution turns dark blue. What do you do now?

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