For diatomic carbon dioxide gas (CO2, molar mass g/mol) at K, calculate the most probable speed .
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21. Kinetic Theory of Ideal Gases
Speed Distribution of Ideal Gases
Problem 71
Textbook Question
At room temperature, it takes approximately 2.45 x 10³ J to evaporate 1.00 g of water. Estimate the average speed of evaporating molecules. What multiple of vrms (at 20°C) for water molecules is this? (Assume Eq. 18–4 holds.)

1
Identify the given data: The energy required to evaporate 1.00 g of water is 2.45 × 10³ J. The mass of 1.00 g of water corresponds to 1.00 × 10⁻³ kg. The root-mean-square (rms) speed of water molecules at 20°C can be calculated using the formula for vᵣₘₛ: , where k is the Boltzmann constant and T is the temperature in kelvins.
Relate the energy required for evaporation to the kinetic energy of the molecules. The energy per molecule can be expressed as , where Q is the total energy (2.45 × 10³ J) and N is the number of molecules in 1.00 g of water. Use Avogadro's number and the molar mass of water (18.015 g/mol) to calculate N.
Calculate the average kinetic energy of a single molecule using the energy per molecule derived in the previous step. The kinetic energy is related to the speed of the molecule by the equation , where m is the mass of a single water molecule and v is the average speed.
Solve for the average speed of the evaporating molecules using the equation , where E is the average kinetic energy of a single molecule. Substitute the values for E and m to find v.
Compare the calculated average speed of the evaporating molecules to the root-mean-square speed (vᵣₘₛ) at 20°C. Use the ratio to determine the multiple of vᵣₘₛ that corresponds to the average speed of the evaporating molecules.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Latent Heat of Vaporization
The latent heat of vaporization is the amount of energy required to convert a unit mass of a substance from liquid to gas at constant temperature. For water, this value is significant because it reflects the energy needed to overcome intermolecular forces during the phase change. In this question, the given energy of 2.45 x 10³ J is the latent heat needed to evaporate 1.00 g of water.
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Average Speed of Molecules
The average speed of molecules in a gas is related to their kinetic energy and temperature. At a given temperature, the average kinetic energy of gas molecules can be calculated using the equation KE = (3/2)kT, where k is the Boltzmann constant and T is the temperature in Kelvin. This concept is crucial for estimating the speed of evaporating water molecules, as it helps relate energy input to molecular motion.
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Root Mean Square Speed (vᵣₘₛ)
The root mean square speed (vᵣₘₛ) is a measure of the average speed of particles in a gas and is calculated using the formula vᵣₘₛ = √(3kT/m), where m is the mass of a molecule. This concept is important for comparing the speed of evaporating molecules to the average speed of water molecules at a specific temperature, allowing for a quantitative understanding of molecular dynamics during evaporation.
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