A tall, narrow graduated cylinder (height = 30 cm) is filled with 250 mL of an…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A tall, narrow graduated cylinder (height = 30 cm) is filled with 250 mL of an aqueous solution that contains a salt. Using a microprobe, a chemist withdraws 5.00 mL aliquots at five evenly spaced depths (0 cm, 7.5 cm, 15 cm, 22.5 cm, 30 cm from the surface). The chloride-ion concentration [Cl−] in each aliquot is determined by titration; the results are shown below.
Depth (cm)
[Cl−] (mol·L−1)
0.0
0.105
7.5
0.106
15.0
0.104
22.5
0.106
30.0
0.105
(a) Using the data, determine whether the sample in the cylinder should be classified as homogeneous or heterogeneous. Fully justify your answer with quantitative reasoning that considers experimental uncertainty.
(b) Suppose the same experiment is repeated with a second cylinder containing an equal volume of a suspension formed by vigorously mixing sand with distilled water. Predict the trend you would expect to observe for [Cl−] or any other suitable analyte as a function of depth after 30 min of settling, and explain how that trend supports your predicted classification of the suspension.
(c) For the salt solution originally studied, propose one additional, independent measurement (other than ion concentration) that could be taken at multiple depths to corroborate your classification. Describe the expected results for a homogeneous sample.