# Number of molecules on human's body

1. Feb 6, 2010

### songoku

1. The problem statement, all variables and given/known data
What is the number of moleules on human's body?

2. Relevant equations

3. The attempt at a solution
Is this some kind of theory question and I just have to memorize it or does it involve calculation?

I have tried searching a bit and found out that a person, about 70 kg, would have 7 x 102 atoms (without calculation)

Thanks

2. Feb 6, 2010

### kuruman

What kind of search did you conduct and to what extent did you filter what you found?

If a person of mass 70 kg has 700 atoms in his/her body then one atom must have a mass of 100 grams. Do you really believe this?

3. Feb 6, 2010

### songoku

Hi kuruman

Ah, I'm sorry. I mean 7 x 1027 atoms. Do I just have to memorize this? But how about if the person is 60 kg?

Thanks

4. Feb 7, 2010

### kuruman

No, you don't have to memorize this. The only number you may have to memorize is Avogadro's number. This is what is known as a "back of the envelope" calculation that needs to be correct to within an order of magnitude, i.e. have the correct power of ten.

You need to make some assumptions first. Since a person is mostly water, H2O, how many water molecules are there in 70 kg worth of water? That should be close enough for an order of magnitude estimate.

5. Feb 7, 2010

### Staff: Mentor

The only problem I see here is whether the question asks about number of molecules IN or ON the human body

6. Feb 7, 2010

### kuruman

Maybe you're right, but I wouldn't know how to calculate the surface area of a human body. Does it increase when one opens one's mouth? What about all those hairs? They have quite a bit of area for the volume they occupy.

7. Feb 7, 2010

### songoku

:rofl:
number of molecules = 70 * 6.02 x 1023/ (18 x 10-3). Thanks

8. Feb 7, 2010

### Doug Huffman

Units please. The answer may be correct but nekkid numbers are meaningless.

About the SA of the human body I recall the 'Rule of Ninths'; head 1, arms 2,3, legs 5, 7, trunk front 8, trunk rear 9.

9. Feb 7, 2010

### songoku

Hi Doug

The unit : molecules. But I think it's ok without unit for this one because "molecules" is not like other units, e.g kg or m/s. I'm not sure how to explain my opinion but I think there are cases where unit is not required. But I am maybe wrong though.

And what is "SA of human body" ? Thanks

10. Feb 7, 2010

### Staff: Mentor

SA = Surface Area.

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