A sample of industrial wastewater is analyzed for its chromium(III) ion concentration…
CHEMISTRY · Multi-part · A Level · Chemistry · Amount of substance · Amount of substance
Problem
A 100.0mL sample of industrial wastewater is analyzed for its chromium(III) ion concentration by a precipitation–back-titration procedure.
Step 1: Excess aqueous sodium hydroxide is added to the sample, precipitating chromium(III) hydroxide.
Step 2: The mixture is filtered, and the precipitate is washed into a new beaker with distilled water. Exactly 50.0mL of 0.500M hydrochloric acid is then added, dissolving the hydroxide precipitate completely.
Step 3: The solution from Step 2, which now contains unreacted hydrochloric acid, is titrated with 0.200M sodium hydroxide solution. The titration requires 27.4mL of the base to reach the endpoint.
Answer the following questions to determine the chromium(III) ion concentration in the original wastewater sample.
(a) Write the balanced net ionic equation for the precipitation of chromium(III) hydroxide in Step 1.
(b) Write the balanced net ionic equation for the reaction that occurs when chromium(III) hydroxide is dissolved by hydrochloric acid in Step 2.
(c) Using the titration data from Step 3, calculate the number of moles of hydrochloric acid that reacted with chromium(III) hydroxide in Step 2.
(d) Determine the molar concentration of chromium(III) ions in the original 100.0mL wastewater sample.