CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
Two beakers, M and N, initially contain equal volumes of water in which lithium bromide (LiBr, a strong electrolyte) and ethylene glycol (HOCH₂CH₂OH, a nonelectrolyte) have been dissolved. A microscopic particulate count for a fixed-volume analysis box in each beaker gives the following initial data:
• Beaker M: 6 Li⁺ ions, 6 Br⁻ ions, 4 ethylene glycol molecules, 180 H₂O molecules
• Beaker N: 6 Li⁺ ions, 6 Br⁻ ions, 4 ethylene glycol molecules, 180 H₂O molecules
The beakers then undergo different physical changes:
• Beaker M is allowed to evaporate until one-half of the water molecules have left the system (no solute escapes).
• Beaker N has an equal volume of pure water added (doubling the number of water molecules) with no loss of solute.
Answer the following questions. Assume thorough mixing and no chemical reactions.
(a) After the processes are complete, determine the new number of Li⁺ ions, Br⁻ ions, ethylene glycol molecules, and water molecules expected inside one analysis box taken from each beaker. Present your results for Beaker M and Beaker N separately.
(b) For each beaker, calculate the factor by which the concentration of LiBr has changed relative to its initial value. Express each factor as a numerical multiple (e.g., “3× more concentrated” or “0.50× the original”). Show your reasoning based on particle counts.
(c) Predict which beaker will now have the higher electrical conductivity and justify your prediction using the updated particulate information.
(d) If the chemist wanted both beakers to return to their original LiBr concentration without removing or adding solute, describe a physical procedure (evaporation or dilution) that could be applied to each beaker and explain, at the particle level, why it would work.