# Homework Help: Kinetic Theory Problem

1. Jan 6, 2010

### dnl65078

1. The problem statement, all variables and given/known data

At standard temperature and pressure (273 K and 1 atmosphere), oxygen has a mass density () of 1.43 kg/m3. The molecular mass (m) of oxygen is 32 u. Calculate the number (n) of molecules in one cm3 of oxygen. (Round to three significant figures.)

2. Relevant equations

R = 8.31 J/molK
kB = 1.38times10^-23 J/K

3.

I thought I had to use the following formula.. n=p(density not pressure)V/m.. When i put it in. it didnt get the right answer

2. Jan 6, 2010

### ideasrule

You have 1.43 kg of oxygen molecules and each molecule weighs 32 u. How many molecules do you have?

There are this many molecules in 1 m^3.

3. Jan 6, 2010

### cepheid

Staff Emeritus
Welcome to PF.

YOu need to show your calculation steps explicitly so that we can try to figure out where you went wrong.

4. Jan 6, 2010

### dnl65078

Do I have to convert u into kg??

5. Jan 6, 2010

### dnl65078

the "u" is whats throwing me off. I've never seen that before

6. Jan 6, 2010

### ideasrule

You don't have to, but I'm curious about how you're going to divide if you don't.

"u" is the atomic mass unit. Use the Internet and check it up!

7. Jan 6, 2010

### cepheid

Staff Emeritus
Why would your teacher ask you question about something that he hadn't taught or defined in class? It stands to reason that you are supposed to know what it means.

Start here:

http://en.wikipedia.org/wiki/Molecular_mass

read the first paragraph until you get to "unified atomic mass unit", which should be a hyperlink. Then you can clink on that link to find out what it is.

8. Jan 6, 2010

### dnl65078

Hey i'm right there with you. He doesnt do anything. but give me a sec.

9. Jan 6, 2010

### dnl65078

Do i have... 2.69x10^25 molecules??

Sorry I'm a total physics idiot.

10. Jan 6, 2010

### cepheid

Staff Emeritus
I thought you already knew what the right answer was? (Otherwise, in your first post, how would you have known that your first attempt was wrong?)

11. Jan 6, 2010

### dnl65078

I am using webassign so when I input the wrong answer it tells me so.

12. Jan 6, 2010

### ideasrule

In one m^3, yes, but the question wants the answer in cm^3.

13. Jan 6, 2010

### dnl65078

So i multiply that answer by 1x10^-6 and that should give me the right answer ?

14. Jan 6, 2010

### ideasrule

yes, that's right