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Acid - Base Reactions

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Do NOT use the Henderson-Hasselbalch equation in solving any of these questions.

1. Ksp = 2 x 10^-19 for LaF3 in water at 25 degrees celcius. How many grams of LaF3 are present in 500 mL of saturated LaF3(aq) at 25 degrees celcius ? The atomic weight of La is 139.

2. Find [H+] in 0.25 M NaC2H3O2 (aq) at 25 degrees celcius given that Ka = 1.8 x 10^-5 for HC2H3O2 (aq) at 25 degrees celcius. You MUST begin by writing the chemical reaction that is at equilibrium.

3. Find [H+] in a solution prepared by dissolving 1.50 g of Ca(OH)2 in 600 mL of water and diluting the solution to a final volume of 800 mL.

4. Find the pH when 45.0 mL of 0.250 M NaOH (aq) is added to 35.0 mL of 0.200M HCl (aq).

5. Find [H+] in a solution prepared by adding 0.40 moles of NaCN to 500 mL of a 0.25M HCN(aq) solution. Assume that there is no volume change when the NaCN is added. For HCN(aq), Ka = 4.9 x 10^-10.

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Acid - Base Reactions
Please show all work.

Do NOT use the Henderson-Hasselbalch equation in solving any of these questions.

1. Ksp = 2 x 10^-19 for LaF3 in water at 25 degrees celcius. How many grams of LaF3 are present in 500 mL of saturated LaF3(aq) at 25 degrees celcius ? The atomic weight of La is 139.

SOLUTION:
Consider the dissociation of LaF3:
LaF3  La+3(aq) + 3F-(aq)
s s 3s
Where: s = solubility of LaF3 which is also equal to the concentration of a saturated solution of LaF3
Solving for s from the Ksp of LaF3:
Ksp = [La+3] [F-]3
Ksp = 2 x 10-19 = (s) (3s)3
2 x 10-19 = 27 s4
s4 = 2 x 10-19 / 27
S4 = 7.41 x 10-21
s = 9.28 x 10-6 M This is the concentration of a saturated solution of LaF3

Solving for the mass of LaF3 in 500 mL saturated solution:
Molarity = M = mol solute / liter solution
9.28 x 10-6 mol/L = x / 0.5 L
x = 4.64 x 10-6 ...

Solution Summary

The expert examines the base reaction for Henderson-Hasselbalch equations in acid.

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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:
https://online.vitalsource.com/#/books/9781269447980/content/id/app04
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:
https://online.vitalsource.com/#/books/9781269447980/content/id/app03
pg 1059

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