Question

unit 3 practice #2

1. based on the structural formulas, which of the following identifies the compound that is more soluble in water and best helps to explain why?

(a) ethane, because the electron clouds of its molecules are more polarizable than those of propanol.
(b) ethane, because its molecules can fit into the spaces between water molecules more easily than those of propanol can.
(c) propanol, because its molecules have a greater mass than the molecules of ethane have.
(d) propanol, because its molecules can form hydrogen bonds with water molecules but those of ethane cannot.

1. a sealed 10.0 l flask at 400 k contains equimolar amounts of ethane and propanol in gaseous form. which of the following statements concerning the average molecular speed of ethane and propanol is true?

(a) the average molecular speed of ethane is less than the average molecular speed of propanol.
(b) the average molecular speed of ethane is greater than the average molecular speed of propanol.
(c) the average molecular speed of ethane is equal to the average molecular speed of propanol.
(d) the average molecular speeds of ethane and propanol cannot be compared without knowing the total pressure of the gas mixture.

Explanation:

1.

• Option A: Ethane is non - polar (symmetrical structure with C - C and C - H bonds of similar electronegativity differences). Propanol has a polar - OH group. Polarizability (related to London dispersion forces) is not the reason for solubility in water (a polar solvent). Water has hydrogen bonding.
• Option B: The ability to fit into spaces (a steric factor) is not the main reason for solubility in water. Solubility in water is mainly due to intermolecular forces like hydrogen bonding.
• Option C: Mass is not the determining factor for solubility in water. Solubility in water is based on intermolecular forces (e.g., hydrogen bonding, dipole - dipole, and London dispersion forces in the order of importance for polar solvents like water).
• Option D: Propanol has an - OH group. The oxygen in the - OH group can form hydrogen bonds with the hydrogen in water molecules (\(H - O - H\)). Ethane (\(C_2H_6\)) is non - polar and cannot form hydrogen bonds with water. Hydrogen bonding is a strong intermolecular force that can significantly increase solubility in a polar solvent like water.

The formula for the root - mean - square speed (\(v_{rms}\)) of a gas is \(v_{rms}=\sqrt{\frac{3RT}{M}}\), where \(R\) is the gas constant (\(8.314\space J/(mol\cdot K)\)), \(T\) is the temperature in Kelvin, and \(M\) is the molar mass of the gas in \(kg/mol\).

Given \(T = 400\space K\) (constant for both gases in the flask). The molar mass of ethane (\(C_2H_6\)) \(M_{ethane}=30\times10^{- 3}\space kg/mol\) and the molar mass of propanol (\(C_3H_8O\)) \(M_{propanol}=60\times10^{-3}\space kg/mol\).

Since \(v_{rms}\propto\frac{1}{\sqrt{M}}\) (when \(T\) is constant), and \(M_{ethane}

Answer:

D. Propanol, because its molecules can form hydrogen bonds with water molecules but those of ethane cannot.

2.