Questions
A student obtains a mixture of the liquids hexane and octane, which are miscible in all proportions. Which of the following techniques would be best for separating the two components of the mixture, and why?
(A) Filtration, because the different densities of the liquids would allow one to pass through the filter paper while the other would not.
(B) Paper chromatography, because the liquids would move along the stationary phase at different rates owing to the difference in polarity of their molecules.
(C) Column chromatography, because the higher molar mass of octane would cause it to move down the column faster than hexane.
(D) Distillation, because the liquids would boil at different temperatures owing to the difference in strength of their intermolecular forces.
▶️Answer/Explanation
Ans: D
To separate a mixture of hexane and octane, both of which are volatile hydrocarbons with similar chemical properties but different boiling points, the most effective technique would be based on the difference
in their boiling points. Let’s analyze the options:
(A) Filtration: Filtration is typically used to separate a solid from a liquid or a precipitate from a solution. However, hexane and octane are both liquids, and they are miscible (mixable) in all proportions.
Their density difference is minimal and not sufficient for effective separation.
(B) Paper chromatography: Paper chromatography separates components based on differences in polarity. While hexane and octane have different boiling points, they have similar polarities since they are
both nonpolar hydrocarbons. Thus, paper chromatography would not effectively separate them.
(C) Column chromatography: Column chromatography separates compounds based on differences in their affinity for a stationary phase (usually silica gel) and their interaction with a mobile phase
(solvent). While octane has a higher molar mass than hexane, their similar chemical properties and nonpolar nature make them unlikely to separate effectively using column chromatography.
(D) Distillation: Distillation is the process of separating components of a mixture based on differences in their boiling points. Hexane has a lower boiling point (69C) compared to octane (126C) due to
differences in the strength of their intermolecular forces. Therefore, distillation would be the most suitable technique to separate hexane and octane effectively.
So, the correct answer is:
(D) Distillation, because the liquids would boil at different temperatures owing to the difference in strength of their intermolecular forces.
Questions
Ne, HF, \(C_{2}H_{6}\), \(CH_{4}\)
Which of the substances listed above has the highest boiling point, and why?
(A) Ne, because its atoms have the largest radius
(B) HF, because its molecules form hydrogen bonds
(C) C2H6, because each molecule can form multiple hydrogen bonds
(D) CH4, because its molecules have the greatest London dispersion forces
▶️Answer/Explanation
Ans: B
The boiling point of a substance depends on the strength of intermolecular forces present in the molecules. Let’s analyze each option:
(A) Ne, because its atoms have the largest radius.
Neon (\(Ne\)) is a noble gas composed of single atoms, which have weak London dispersion forces. These forces are generally weaker than other intermolecular forces, such as hydrogen bonding or dipole-dipole interactions.
(B) HF, because its molecules form hydrogen bonds.
Hydrogen fluoride (\(HF\)) is a polar molecule that can form hydrogen bonds between its hydrogen and fluorine atoms. Hydrogen bonding is a strong intermolecular force, leading to higher boiling points compared to substances with only London dispersion forces.
(C) \(C_{2}H_{6}\), because each molecule can form multiple hydrogen bonds.
Ethane (\(C_{2}H_{6}\)) is a nonpolar molecule and cannot form hydrogen bonds. Although it has London dispersion forces, these forces are generally weaker than hydrogen bonding.
(D) \(CH_{4}\), because its molecules have the greatest London dispersion forces.
Methane (\(CH_{4}\)) is a nonpolar molecule, and its only intermolecular forces are London dispersion forces. While London dispersion forces can contribute to boiling point, they are generally weaker than hydrogen bonding.
Based on the analysis, option (B) HF, because its molecules form hydrogen bonds, is the most accurate. Hydrogen bonding is a stronger intermolecular force compared to London dispersion forces, leading to a higher boiling point for hydrogen fluoride compared to the other substances listed.
Questions
\(Cu_{(s)}+4HNO_{3}(aq)\rightarrow Cu(No_{3})_{2}(aq)+2No_{2(g)}+2H_{2}o_{(l)}\)
Each student in a class placed a \(2.00 \mathrm{~g}\) sample of a mixture of \(\mathrm{Cu}\) and \(\mathrm{Al}\) in a beaker and placed the beaker in a fume hood. The students slowly poured \(15.0 \mathrm{~mL}\) of \(15.8 \mathrm{M} \mathrm{HNO}_3(a q)\) into their beakers. The reaction between the copper in the mixture and the \(\mathrm{HNO}_3(a q)\) is represented by the equation above. The students observed that a brown gas was released from the beakers and that the solutions turned blue, indicating the formation of \(\mathrm{Cu}^{2+}(a q)\). The solutions were then diluted with distilled water to known volumes.
To determine the number of moles of Cu in the sample of the mixture, the students measured the absorbance of known concentrations of Cu(NO3)2(aq) using a spectrophotometer. A cuvette filled with some of the solution produced from the sample of the mixture was also tested. The data recorded by one student are shown in the table above. On the basis of the data provided, which of the following is a possible error that the student made?
(A) The Cu(NO3)2(aq) from the sample of the mixture was not diluted properly.
(B) The spectrophotometer was calibrated with tap water instead of distilled water.
(C) The student labeled the cuvettes incorrectly, reversing the labels on two of the solutions of known concentration.
(D) The spectrophotometer was originally set to an inappropriate wavelength, causing the absorbance to vary unpredictably.
▶️Answer/Explanation
Ans: C
- option \(C\) is correct because the rise in absorbance is linearly increasing but the labels are not correct.
The correct order is.
- With rise in concenterat absorbance increases.
▶️Answer/Explanation
Ans: A
Absorption of photons in the visible range will cause transition of electrons between electron energy levels.