Basic Kinetic theory: How many molecules are there?

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Homework Help Overview

The discussion revolves around a problem in kinetic theory, specifically concerning the number of molecules in water vapor at equilibrium with liquid water at a given temperature and pressure. The original poster presents a scenario involving calculations related to the volume occupied by water vapor and the impacts of vapor molecules on the liquid surface.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants explore the calculation of the number of molecules in a given volume of water vapor and question the unit conversions involved in the problem. There is an attempt to apply the ideal gas law and discuss the implications of the provided values.

Discussion Status

The discussion is ongoing, with participants providing insights into unit conversions and calculations. Some guidance has been offered regarding the number of molecules in 1 gram of water, and there is acknowledgment of potential errors in unit conversions.

Contextual Notes

There is a noted confusion regarding the conversion between cubic centimeters and cubic meters, which may affect the calculations. The problem also involves assumptions about the behavior of gases at equilibrium and the properties of water vapor.

Lagraaaange
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Homework Statement


A closed vessel contains liquid water at equilibrium with its vapor at 100C and 1 atm. 1 gram of water vapor occupies 1670cm^3. Hvap = 2250Jg^-1. How many molecules are there per cm^3 of vapor?How many vapor molecules strike each cm^2 of liquid surface per second?

Homework Equations


PV = NKT
Q = lm

The Attempt at a Solution


N = PV/KT = 3.3E26 molecules of vapor
I used P = 1.013 E5 N m^-2
V = 16.7m^3
T = 373
Not sure how to get how much water there is.
Answers are: 2E19 molecules cm^-2 ; 3.3E23 impacts per second
 
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How many water molecules are there in 1 gm of H2O? You are given the volume of 1 gm, so given these two, you should be able to calculate the number of molecules in 1 cm^3.
 
Lagraaaange said:

Homework Statement


A closed vessel contains liquid water at equilibrium with its vapor at 100C and 1 atm. 1 gram of water vapor occupies 1670cm^3. Hvap = 2250Jg^-1. How many molecules are there per cm^3 of vapor?How many vapor molecules strike each cm^2 of liquid surface per second?

Homework Equations


PV = NKT
Q = lm

The Attempt at a Solution


N = PV/KT = 3.3E26 molecules of vapor
I used P = 1.013 E5 N m^-2
V = 16.7m^3
T = 373
Not sure how to get how much water there is.
Answers are: 2E19 molecules cm^-2 ; 3.3E23 impacts per second
BTW, 1670 cm3 ≠ 16.7 m3. Check your unit conversions again, since 1 m3 ≠ 100 cm3. :frown:
 
SteamKing said:
BTW, 1670 cm3 ≠ 16.7 m3. Check your unit conversions again, since 1 m3 ≠ 100 cm3. :frown:
Wow. Thanks
 

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