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pH, Buffers, Weak Acid Ka, and Kb Values

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Imagine that you are in chemistry lab and need to make 1.00L of a solution with a pH of 2.50. You have in front of you:

- 100mL of 7.00x10-2 M HCl

- 100mL of 5.00x10-2 M NaOH, and

- Plenty of distilled water.

You start to add HCl to a beaker of water when someone asks you a question. When you return to your dilution, you accidentally grab the wrong cylinder and add some NaOH. Once you realize your error, you assess the situation. You have 82.0 mL of HCl and 85.0 mL of NaOH left in their original containers.

Assuming the final solution will be diluted 1.00L, how much more HCl should you add to achieve the desired pH?

Express your answer numerically in milliliters to three significant figures.

_________________________ mL


You need to produce a buffer solution that has a pH of 5.42. You already have a solution that contains 10 mmol (millimoles) of acetic acid. How many millimoles of acetate (the conjugate base of acetic acid) will you need to add to this solution? The pKa of acetic acid is 4.74.

Express your answer numerically in millimoles to three significant figures:

_______________________ mmol acetate


A new potential heart medicine, code-named X-281, is being tested by a pharmaceutical company, Pharmapill. As a research technician at Pharma-pill, you are told that X-281 in a monoprotic weak acid, but because of security concerns, the actual chemical formula must remain top secret. The company is interested in the drug's Ka value because only the dissociated form of the chemical is active in preventing cholesterol buildup in arteries.

To find the pKa of X0281, you prepare a 0.075 M test solution of X-281. The pH of the solution is determined to be 2.70. What is the pKa of X-281?

Express your answer numerically.

pKa = _____________________________________


Given that

- Ka for HCN is 4.9 x 10-10 and
- Kb for NH3 is 1.8 x 10 -5,


- Kb for CN-
- Ka for NH4+

Enter the Kb value for CN- followed by the Ka value for NH4+, separated by a comma, using two significant figures.


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Solution Summary

This solution describes:

1) How to use specific volumes of strong acids and bases to make a solution with a specific pH.

2) How to make a buffer solution.

3) How to find the Ka based on the concentration and pH of a weak acid

4) How to find Kb based on a Ka value.

Similar Posting

Acid-Base Reactions, Buffer and Thermodynamics

I am stuck on a few chemistry questions. Please help.

17.16) Use information from Appendix D to calculate the pH of (a) a solution that is 0.250 M in sodium formate (HCOONa) and 0.100 M in formic acid (HCOOH); (b) a solution that is 0.510 M in pyridine (C5H5N) and 0.450 M in pyridinium chloride (C5H5NHCl); (c) a solution that is made by combining 55 mL of 0.050 M hydrofluoric acid with 125 mL of 0.10 M sodium fluoride.

For Appendix D see:
pg 1062

17.18) (a) Calculate the percent ionization of 0.125 M lactic acid (Ka = 1.4 × 10−4). (b) Calculate the percent ionization of 0.125 M lactic acid in a solution containing 0.0075 M sodium lactate.

17.28) A buffer contains 0.15 mol of propionic acid (C2H5COOH) and 0.10 mol of sodium propionate (C2H5COONa) in 1.20 L. (a) What is the pH of this buffer? (b) What is the pH of the buffer after the addition of 0.01 mol of NaOH? (c) What is the pH of the buffer after the addition of 0.01 mol of HI?

17.46) Consider the titration of 30.0 mL of 0.050 M NH3 with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (a) 0 mL, (b) 20.0 mL, (c) 59.0 mL, (d) 60.0 mL, (e) 61.0 mL, (f) 65.0 mL.

17.66) Using the value of Ksp for Ag2S, Ka1 and Ka2 for H2S, and Kf = 1.1 × 105 for AgCl2−, calculate the equilibrium constant for the following reaction:
Ag2S(s) + 4 Cl−(aq) + 2 H+(aq) ⇌ 2 AgCl2−(aq) + H2S(aq)

17.72) A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2+ and 0.010 M in Sr2+. (a) What concentration of SO42− is necessary to begin precipitation? (Neglect volume changes. BaSO4: Ksp = 1.1 × 10−10; SrSO4: Ksp = 3.2 × 10−7.) (b) Which cation precipitates first? (c) What is the concentration of SO42− when the second cation begins to precipitate?

17.89) A sample of 0.1687 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.1150 M NaOH. The acid required 15.5 mL of base to reach the equivalence point. (a) What is the molecular weight of the acid? (b) After 7.25 mL of base had been added in the titration, the pH was found to be 2.85. What is the Ka for the unknown acid?


19.26) The element gallium (Ga) freezes at 29.8°C, and its molar enthalpy of fusion is ΔHfus = 5.59 kJ/mol. (a) When molten gallium solidifies to Ga(s) at its normal melting point, is ΔS positive or negative? (b) Calculate the value of ΔS when 60.0 g of Ga(l) solidifies at 29.8°C.

19.44) Predict the sign of ΔSsys for each of the following processes: (a) Molten gold solidifies. (b) Gaseous Cl2 dissociates in the stratosphere to form gaseous Cl atoms. (c) Gaseous CO reacts with gaseous H2 to form liquid methanol, CH3OH. (d) Calcium phosphate precipitates upon mixing Ca(NO3)2(aq) and (NH4)3PO4(aq).

19.54) Calculate ΔS° values for the following reactions by using tabulated S° values from Appendix C. In each case explain the sign of ΔS°.
(a) HNO3(g) + NH3(g) → NH4NO3(s)
(b) 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g)
(c) CaCO3(s, calcite) + 2HCl(g) → CaCl2(s) + CO2(g) + H2O(l)
(d) 3 C2H6(g) → C6H6(l) + 6 H2(g)

19.66) From the values given for ΔH° and ΔS°, calculate ΔG° for each of the following reactions at 298 K. If the reaction is not spontaneous under standard conditions at 298 K, at what temperature (if any) would the reaction become spontaneous?

19.86) The Kb for methylamine (CH3NH2) at 25°C is given in Appendix D. (a) Write the chemical equation for the equilibrium that corresponds to Kb. (b) By using the value of Kb, calculate ΔG° for the equilibrium in part (a). (c) What is the value of ΔG at equilibrium? (d) What is the value of ΔG when [H+] = 6.7 × 10-9 M, [CH3NH3+] = 2.4 × 10-3 M, and [CH3NH2] = 0.098 M?

19.97) Consider the following three reactions:
(i) Ti(s) + 2 Cl2(g) → TiCl4(g)
(ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g)
(iii) BaO(s) + CO2(g) → BaCO3(s)
(a) For each of the reactions, use data in Appendix C to calculate ΔH°, ΔG°, and ΔS° at 25°C. (b) Which of these reactions are spontaneous under standard conditions at 25°C? (c) For each of the reactions, predict the manner in which the change in free energy varies with an increase in temperature.

For appendix C See:
pg 1059

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