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Ch.11 - Liquids, Solids & Intermolecular Forces
Tro - Chemistry: A Molecular Approach 4th Edition
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Tutti i libri di testoTro 4th EditionCh.11 - Liquids, Solids & Intermolecular ForcesProblema 57
Capitolo 11, Problema 57

The human body obtains 915 kJ of energy from a candy bar. If this energy were used to vaporize water at 100.0 °C, how much water (in liters) could be vaporized? (Assume the density of water is 1.00 g/mL.)

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Identify the energy required to vaporize water, which is the heat of vaporization (\(\Delta H_{vap}\)) for water at 100.0 °C. This value is typically 40.7 kJ/mol.
Convert the energy obtained from the candy bar from kilojoules to joules. Since 1 kJ = 1000 J, multiply 915 kJ by 1000 to get the energy in joules.
Calculate the number of moles of water that can be vaporized using the energy from the candy bar. Use the formula: \(\text{moles of water} = \frac{\text{energy in joules}}{\Delta H_{vap} \times 1000}\), where \(\Delta H_{vap}\) is in kJ/mol.
Convert the moles of water to grams using the molar mass of water (18.02 g/mol). Use the formula: \(\text{mass of water (g)} = \text{moles of water} \times 18.02\, \text{g/mol}\).
Convert the mass of water in grams to volume in liters. Since the density of water is 1.00 g/mL, the volume in mL is equal to the mass in grams. Convert mL to liters by dividing by 1000.

Concetti chiave

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Heat of Vaporization

The heat of vaporization is the amount of energy required to convert a unit mass of a liquid into vapor without a change in temperature. For water, this value is approximately 2260 J/g at 100 °C. Understanding this concept is crucial for calculating how much water can be vaporized using a given amount of energy.
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Heat Capacity

Energy Conversion

Energy conversion involves changing energy from one form to another. In this context, the energy obtained from the candy bar (915 kJ) must be converted into joules (1 kJ = 1000 J) to match the units used for the heat of vaporization. This step is essential for accurately determining the amount of water that can be vaporized.
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Conversion Factors

Density of Water

Density is defined as mass per unit volume, and for water, it is typically 1.00 g/mL at standard conditions. This property allows for the conversion between mass and volume, which is necessary when calculating how many liters of water can be vaporized based on the mass of water that corresponds to the energy provided.
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Density via Water Displacement
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This table displays the vapor pressure of ammonia at several different temperatures. Use the data to determine the heat of vaporization and normal boiling point of ammonia.

Temperature (K) Pressure (torr)

200 65.3

210 134.3

220 255.7

230 456.0

235 597.0

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Which evaporates more quickly: 55 mL of water (H2O) in a beaker or 100 mL of acetone [(CH3)2CO] in an identical beaker under identical conditions? Is the vapor pressure of the two substances different? Explain.

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