A student attempts to prepare a saline solution by placing 5.85 g of solid NaCl into…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A student attempts to prepare a saline solution by placing 5.85 g of solid NaCl into a 1.00 L volumetric flask and adding water until the bottom of the meniscus reaches the calibration line. Before swirling, the student quickly removes a 25.0 mL aliquot from the top of the flask for analysis. After thorough mixing, the student removes a second 25.0 mL aliquot from the top and analyzes both samples for [Na+].
The measured sodium-ion concentrations are:
• Before mixing: 0.60 mol L−1
• After mixing: 0.100 mol L−1
(a) Calculate the theoretical molarity of the solution assuming complete dissolution of the NaCl and perfect homogeneity.
(b) Compare the measured concentrations for the two aliquots with the theoretical value from part (a). Discuss what the comparison reveals about the distribution of solute before and after mixing.
(c) Based on your analysis, state whether the contents of the flask are homogeneous or heterogeneous (i) before mixing and (ii) after mixing. Provide evidence for each claim.
(d) Propose one procedural modification that would have allowed the student to obtain a sample whose composition matched the theoretical value the first time.