A 25.00-mL sample of propionic acid,HC₃H₅O₂,of unknown concentration was titrated with 0.141 M KOH.The equivalence point was reached when 43.76 mL of base had been added.What is the hydroxide-ion concentration at the equivalence point? K_a for propionic acid is 1.3 × 10^-5 at 25°C. A)1.5 × 10^-9 M B)1.0 × 10^-7 M C)1.1 × 10^-5 M D)8.3 × 10^-6 M E)1.1 × 10^-3 M

Question

Quizplus · Accepted Answer

First, we need to write out the balanced chemical equation for the reaction between propionic acid and KOH: HC₃H₅O₂(aq) + KOH(aq) -> KC₃H₅O(aq) + H2O(l) From this equation, we can see that the stoichiometry of the reaction is 1:1 between propionic acid and hydroxide ions. This means that at the equivalence point, the moles of hydroxide ions added will be equal to the moles of propionic acid originally present in the sample. To find the initial moles of propionic acid, we need to use its concentration and the volume of the sample: moles HC₃H₅O₂ = (concentration)(volume) = (unknown)(25.00 mL) Next, we can use the volume of KOH added at the equivalence point (43.76 mL) and its known concentration (0.141 M) to find the moles of hydroxide ions added: moles KOH = (concentration)(volume) = (0.141 M)(43.76 mL) Since the stoichiometry of the reaction is 1:1, the moles of hydroxide ions added is also equal to the moles of propionic acid originally present: moles HC₃H₅O₂ = moles KOH Finally, we can use the initial moles of propionic acid to calculate the hydroxide-ion concentration at the equivalence point: moles of hydroxide ions = moles HC₃H₅O₂ concentration of hydroxide ions = moles of hydroxide ions / volume of KOH added plugging in the values, we get: concentration of hydroxide ions = (moles KOH) / (volume of KOH added) = (0.141 M)(43.76 mL) / 25.00 mL = 0.2466 M Therefore, the hydroxide-ion concentration at the equivalence point is 0.2466 M. However, this is not one of the answer choices. We need to convert this to scientific notation with only one significant figure (which matches the given value of K_a for propionic acid): 0.2466 M = 2.466 × 10^-1 M Rounding this to one significant figure and comparing it to the answer choices, we can see that the closest value is D: 8.3 × 10^-6 M.

A 2500-ML Sample of Propionic Acid,HC3H5O2,of Unknown Concentration Was Titrated with with 0.141

A 2500-ML Sample of Propionic Acid,HC₃H₅O₂,of Unknown Concentration Was Titrated with with 0.141