# Basic Kinetic theory: How many molecules are there?

## 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?

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

phyzguy
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.

SteamKing
Staff Emeritus
Homework Helper

## 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?

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. BTW, 1670 cm3 ≠ 16.7 m3. Check your unit conversions again, since 1 m3 ≠ 100 cm3. Wow. Thanks