Question

Explain, in your own words, (a) how to determine experimentally whether a pure substance is an electrolyte or a nonelectrolyte. (b) why a cold glass of beer goes "flat" upon warming. (c) why the molality of a solute is ordinarily larger than its mole fraction. (d) why the boiling point is raised by the presence of a solute.

Short answer

Question: Explain how to determine if a substance is an electrolyte or a nonelectrolyte experimentally and the effect of a solute on the boiling point of a solution. Answer: To determine if a substance is an electrolyte or a nonelectrolyte experimentally, dissolve it in water and test its ability to conduct electricity using a simple circuit with a light bulb. If the light bulb lights up, the substance is an electrolyte; if not, it's a nonelectrolyte. The presence of a solute in a solution raises its boiling point due to boiling point elevation, which occurs because the solute disrupts the solvent's vapor pressure, requiring more energy to reach the same vapor pressure as the pure solvent.

Step-by-step solution

(a) Determining Electrolytes and Nonelectrolytes Experimentally

An electrolyte is a substance that, when dissolved in water, conducts electricity due to the presence of free ions. A nonelectrolyte does not conduct electricity because it does not dissociate into ions. To determine experimentally if a substance is an electrolyte or a nonelectrolyte, perform the following steps: 1. Dissolve a small amount of the substance in water to create a solution. 2. Connect a light bulb or LED to a battery and complete the circuit with two electrodes. Make sure the electrodes are not touching each other. 3. Place the electrodes in the solution. If the light bulb or LED lights up, the substance is an electrolyte as it conducts electricity. If the light bulb or LED remains off, the substance is a nonelectrolyte and does not conduct electricity.

(b) Going 'Flat' Upon Warming

A cold glass of beer contains dissolved carbon dioxide (CO2) gas, which contributes to the overall fizzy and carbonated taste. When the beer is warmed up, the solubility of CO2 decreases, which means less CO2 can be dissolved in the liquid. As a result, the CO2 escapes as gas bubbles, causing the beer to lose its fizziness and taste "flat."

(c) Comparing Molality and Mole Fraction

Molality is the ratio of the moles of solute to the mass of solvent (in kilograms), while the mole fraction is the ratio of moles of solute to the total moles in the solution. Molality is generally larger than the mole fraction of a solute because molality is based on mass, while mole fraction is based on moles. Since the mass of a solvent is typically larger than the combined mass of solute and solvent, the molality of the solute will be larger than its mole fraction.

(d) Boiling Point Elevation

The boiling point of a solution is raised by the presence of a solute due to the phenomenon known as boiling point elevation. When a non-volatile solute is added to a solvent, it disrupts the solvent's vapor pressure, making it more difficult for the solvent's molecules to escape into the vapor phase. As a result, the solution requires more energy (in the form of heat) to achieve the same vapor pressure as the pure solvent. This increases the boiling point of the solution compared to the pure solvent.