# Homework Help: Atomic mass unit

1. Apr 25, 2005

### kat_r

how do you convert the atomic mass unit into kgs?

2. Apr 25, 2005

### xanthym

Atomic Mass Unit - Kilogram
(Ref: NIST)

{1 amu ≅ 1.66053886 x 10(-27) kg}
{Standard Uncertainty: 0.00000028 x 10(-27) kg}
{Relative Standard Uncertainty: 1.7 x 10(-7)}

(Corrected Thanks To Curious3141, et. al.)
~~

Last edited: Apr 25, 2005
3. Apr 25, 2005

### Curious3141

I'm afraid that isn't correct, xanthym.

1 atomic mass unit = 1.660538 × 10^(-27) kilograms

It should be the same order of magnitude as a proton's mass. What you quoted is electron mass.

4. Apr 25, 2005

### Night Owl

Curious 3141 is right; that first post is wrong. He gave the mass of an electron. Curious 3141's value should be right (I remember it being 1.66 x 10^-27 kg = 1 amu)

If you wanted to derive it, you could remember that 1 amu is defined as 1/12 the mass of a carbon 12 atom, which, by definition of the mole, I believe, has a molar mass of 12 g/mol. So if you have 6.022 x 10^23 atoms (1 mole of atoms) of Carbon-12, you'll have (6.022 x 10^23 atoms)(12 amu/atom) = (12 g)(1 kg/1000 g)

Which comes out to: 7.2264 x 10^24 amu = 0.012 kg

Divide both sides by 7.2264 x 10^24, and you get:

1 amu = 1.661 x 10^-27 kg <--- with significant figures.

I'm pretty sure the reasoning behind that method of derivation is sound. In any case, it gets you the right result. The only reason I wonder about it is because, I know that carbon-12 is defined as having exactly 12 amu, and I'm pretty sure that one mole of atoms is defined as the number of atoms required to achieve a mass, in grams, whose numerical value is equal to the numerical value of amu (so a mole of carobon-12, which is 12 amu exactly, will have a mass of 12 grams exactly).

Anyway. Yeah. Curious is right.

Last edited: Apr 25, 2005
5. Apr 25, 2005

### dextercioby

It's interesting that he edited the message 9 minutes later...

When u give physical constants with nonexact values,don't use the "=" sign,unless you give the uncertainty in that value...Use the approximately equal sign.

[tex] 1\mbox{amu}\simeq 1.66\cdot 10^{-27}\mbox{Kg} [/itex]

Daniel.

6. Apr 25, 2005

### xanthym

(Original Msg Corrected Thanks To Curious3141, et. al.):

Atomic Mass Unit - Kilogram
(Ref: NIST)

{1 amu ≅ 1.66053886 x 10(-27) kg}
{Standard Uncertainty: 0.00000028 x 10(-27) kg}
{Relative Standard Uncertainty: 1.7 x 10(-7)}

~~

7. Apr 25, 2005

### Night Owl

I said it had significant figures, thus implying it was not exact.

I don't exactly have one of those squiggly approximately equal-to signs on my keyboard, and I don't know how to make the images you seem to be able to use on the forum here, since I'm new.

And if you're typing, you should spell out your words like "you" instead of writing "u," but I knew what you meant. Nonetheless, your comment has been noted.

Last edited: Apr 25, 2005
8. Apr 25, 2005

### dextercioby

Seems familiar to u,alias you...? Oh,"~" is a good sign to use,unless u know how to edit in LaTex...

Daniel.