
Concept explainers
(a)
Interpretation:
The molar mass of acid and Ka of the unknown acid has to be calculated.
Concept introduction:
- Molar mass is the ratio of mass of given substance to that number of moles of compound.
- Molar mass = Amount of given substanceNumber of moles
- Acid dissociation constant Ka is defined as measure of acid strength in a given solution
- Ka = [A-][H+][HA]
To calculate: the molar mass of monoprotic acid
(a)

Answer to Problem 17.22QP
The molar mass of monoprotic acid is 1.10 × 102g/mol.
Explanation of Solution
Amount of monoprotic acid is 0.1276 g
Concentration of NaOH is 0.0633 M
Calculate the number of moles of monoprotic acid
HA(aq) + NaOH(aq) → NaA(aq) + H2O(l)
Number of moles of monoprotic acid = 18.4 mL × 0.0633 mol1000 mL = 0.00116 mol
In this reaction one mole of monoprotic acid needs to neutralise one mole of NaOH. The number of moles of monoprotic acid can be calculated using volume and given concentration of the formic acid.
Calculate the molar mass of monoprotic acid
Molar mass = Amount of given substanceNumber of molesMolar mass = 0.1276 g HA0.00116 mol HA = 1.10 × 102g/mol
The molar mass of monoprotic acid can be calculated on dividing given weight of monoprotic acid by number of moles of monoprotic acid.
(b)
Interpretation:
The molar mass of acid and Ka of the unknown acid has to be calculated.
Concept introduction:
- Molar mass is the ratio of mass of given substance to that number of moles of compound.
- Molar mass = Amount of given substanceNumber of moles
- Acid dissociation constant Ka is defined as measure of acid strength in a given solution
- Ka = [A-][H+][HA]
To calculate: the number of moles of NaOH and concentration of hydrogen ion
(b)

Answer to Problem 17.22QP
The Ka of the monoprotic acid is 1.6 × 10-6
Explanation of Solution
Amount of monoprotic acid is 0.1276 g
Concentration of NaOH is 0.0633 M
10 mL × 0.0633 mol1000 mL = 6.33 × 10-4
The reaction is as follows, HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) Initial concentration(M): 0.0016 6.33 × 10-4 0 Change in concentration (M): 6.33 × 10-4 6.33 × 10-4 +6.33 × 10-4 Final concentration (M): 5.2 × 10-4 0 6.33 × 10-4The concentration of HA and A- are [HA] = 5.2 ×10-4 mol0.0350 L = 0.015 M [A-] = 6.33 ×10-4 mol0.0350 L = 0.0181 M pH = -log[H+] 5.87 = -log[H+] [H+] = 10-pH = 10-5.87 = 1.35 × 10-6 M
The number of moles of sodium of hydroxide can be calculated using the given concentration divided by thousand. The weak acid reacts with NaOH and the concentrations vary from the obtained concentrations, strength of hydrogen ion can be determined.
Calculate the Ka of the monoprotic acid
The equilibrium is as follows, HA(aq) → H+(aq) + A-(aq) Initial concentration(M): 0.0015 0 0.0181 Change in concentration (M): 1.35 × 10-6 +1.35 × 10-6 +1.35 × 10-6 Final concentration (M): 0.015 1.35 × 10-6 0.0181 Ka = [A-][H+][HA] = (1.35 × 10-6)(0.0181)0.015 = 1.6 × 10−6
The Ka can be calculated by using concentrations of reactants and products. By doing simple mathematical calculations the acid dissociation constant can be determined.
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Chapter 17 Solutions
EBK GENERAL CHEMISTRY: THE ESSENTIAL CO
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