Short free-response question
Preparing and separating solutions
- Unit 3
- 4 points
- About 9 minutes
You can use a calculator on this question, just like on exam day.
A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.
The question and its sources
A student works with several solutions and mixtures in the laboratory.
Thin-layer chromatography
The student spots a mixture of two dyes, P and Q, on a plate coated with silica, a very polar solid. The plate is developed with hexane, a nonpolar solvent.
After developing, dye P has traveled 0.80 of the distance the solvent traveled (Rf = 0.80), and dye Q has traveled 0.25 of that distance (Rf = 0.25).
Source: Hypothetical data
Suggested time: 9 minutes
Your answers are saved in this browser as you type.
Part (a)
1 pointCalculate the mass of KCl(s) needed to prepare 250.0 mL of 0.150 M KCl(aq).
0 / 2,500 characters
Part (b)
1 pointDescribe a particle-level drawing of the KCl solution that shows how water molecules interact with one K⁺ ion and with one Cl⁻ ion. Specify which end of each water molecule points toward each ion, and justify the orientation.
0 / 2,500 characters
Part (c)
1 pointWhich dye, P or Q, is more polar? Justify your answer in terms of the interactions of the dyes with the silica and with the hexane.
0 / 2,500 characters
Part (d)
1 pointSolid iodine, I₂, dissolves readily in hexane, C₆H₁₄, but only very slightly in water. Explain this difference in terms of intermolecular forces.
0 / 2,500 characters
Checking scoring…
Scoring it yourself shows you the rubric, examples and a model answer. Try writing your answer first.