A student attempts to sketch a particulate-level diagram representing an aqueous…
CHEMISTRY · Subjective · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A student attempts to sketch a particulate-level diagram representing an aqueous solution prepared by dissolving solid Na2SO4 in water. The student’s diagram is described verbally below.
Box description (intended to represent a fixed microscopic volume of the solution):
• 25 water molecules are shown. Each water molecule is drawn as one large red sphere (oxygen) with two smaller white spheres (hydrogen) attached. In every water molecule, the two white spheres are oriented so that they point directly toward the oxygen of a neighboring water molecule.
• 4 intact Na2SO4 formula units, each drawn as a cluster containing one purple sphere labeled “Na2” and one yellow–green tetrahedral unit labeled “SO4,” are floating in the box. Water molecules surround these clusters but are not oriented in any specific way.
• 3 isolated sulfate ions (yellow–green tetrahedral shape labeled “SO4²⁻”) are present. Around each sulfate ion the water molecules are oriented so that their hydrogens (white spheres) point toward the central sulfur atom.
• 8 isolated sodium ions (purple spheres labeled “Na⁺”) are present. Around each sodium ion the water molecules are oriented so that their oxygen atoms (red spheres) point toward the Na⁺ ion.
(a) Identify THREE distinct errors in the student’s diagram related to accepted conventions for particulate representations of ionic solutions. List the errors clearly.
(b) For EACH error you identified in part (a), explain why the representation is incorrect by referring to fundamental ideas about dissociation, charge balance, or ion–dipole interactions.
(c) Describe in words how the corrected particulate diagram for this Na2SO4(aq) solution should appear. Your description should address the proper dissociation products, the correct relative numbers of ions, and the correct orientation of surrounding water molecules for BOTH the cations and the anions.