# Homework Help: Need help finding center of mass of a molecule.

1. Mar 24, 2013

### Sneakatone

Figure below shows the shape of a nitric acid (HNO3)molecule and its dimensions. Treating the atoms as particles, findthe center of mass of this molecule.

a)I know that xcm=m1x1+m2x1+...../Total mass.
The total mass in this case is 63 but based on the picture I dont know which mass I should plug in for the x values.

b)ycm=0 because there is no y axis.

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2. Mar 24, 2013

### voko

The mass for every x value is the mass of the atom at that position.

What do you mean by "there is no y axis"?

3. Mar 24, 2013

### Sneakatone

so you mean for the 1st one 0.1 nm*1(mass of hydrogen)+0.141nm*16(mass of oxygen)/63?

theres no y axis means there isnt an angle that goes 90 degrees.

4. Mar 24, 2013

### voko

There is a Y axis. And it is even clearly labeled as such.

5. Mar 24, 2013

### Sneakatone

a) 0.1 nm*1(mass of hydrogen)+0.141nm*16(mass of oxygen)+0.282*14(mass of nitrogen)/63
(should I use the two lengths by nitrogen 0.141nm+0.141nm or just 0.141nm*14g)?

b) the y axis has a length of zero so when multiplied with the mass of oxygen it becomes zero.

6. Mar 24, 2013

### voko

What are the distances measured from?

An axis cannot have a length of zero. An axis is infinite by definition.

7. Mar 24, 2013

### Sneakatone

the distances are measures from the 130 degree angle.

8. Mar 24, 2013

### voko

And I would think the distances are measured from the origin of the coordinate system, which is at the intersection of the X and Y axes. I advise that before going further, you should find out the coordinates of all the atoms in the molecule.

9. Mar 24, 2013

### Sneakatone

if the origin is O where the y axis is labeled then coordinates are ,H(0,-0.1),N(0,0.141)

10. Mar 24, 2013

### voko

The molecule has 5 atoms.

11. Mar 24, 2013

### Sneakatone

H(-0.1,0),o(0,0),n(0.141,0), o(0.2,0.12),o(0.2,-0.12)

12. Mar 24, 2013

### voko

I assume the first number in all the pair is the Y coordinate, correct? It is unusual, but permissible. The first three pairs are OK. The last two, however, are completely wrong.

13. Mar 24, 2013

### Sneakatone

fot the last 2 I did .141cos65 ans added to .141

14. Mar 24, 2013

### voko

I am sorry, I have misread your message. The coordinates are correct. Now you have all the X's and all the Y's to compute the c. m.

15. Mar 26, 2013

### Sneakatone

thanks alot! , I plugged em into the equation and ended up with 0.131 for x and 0 for y.