A beaker contains exactly 18.0 g of NaCl(s) and 18.0 g of urea, CO(NH2)2(s)
CHEMISTRY · Subjective · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A beaker contains exactly 18.0 g of NaCl(s) and 18.0 g of urea, CO(NH2)2(s), both added to enough water to make a total solution volume of 1.00 L at 25 °C. A beginning chemistry student attempts to depict this mixed solution at the particulate level in a square box meant to represent a microscopic slice of the bulk liquid. The student’s diagram is described below.
Student diagram description (fixed-volume box):
• 60 water molecules are shown.
• 4 intact NaCl formula units are present.
• 8 Na⁺ ions and 8 Cl⁻ ions are present, each surrounded by 4 water molecules oriented with the oxygen atoms toward Na⁺ and with the hydrogen atoms toward Cl⁻, respectively.
• 12 intact urea molecules are present, with no particular orientation or hydrogen bonding shown for surrounding water molecules.
(a) Based on the molar masses NaCl = 58.5 g mol⁻¹ and CO(NH2)2 = 60.1 g mol⁻¹, calculate the total number of moles of each solute added to the beaker.
(b) Using your results from part (a), determine the expected mole ratio of NaCl to urea particles after complete dissolution. (Assume NaCl dissociates fully into Na⁺ and Cl⁻ while urea remains molecular.)
(c) Evaluate whether the relative numbers of Na⁺ ions, Cl⁻ ions, and urea molecules shown in the student’s diagram are consistent with the mole ratio you calculated in part (b). Provide quantitative support for your evaluation.
(d) Identify TWO distinct representational errors in the student’s diagram other than any numerical mismatch discussed in part (c). For each error, explain what correction is needed and why it is necessary for an accurate depiction of this mixed electrolyte–nonelectrolyte solution at the particulate level.
(e) Describe how you would redraw the box so that it correctly represents the mixed solution, including (i) the appropriate numbers of Na⁺, Cl⁻, and urea particles relative to 60 water molecules, and (ii) the proper orientation of water molecules around EACH type of solute particle.