How to Convert Amu to Kg

Carbon is used as the basis for defining an atomic mass unit (amu).

An atomic mass unit (amu) is defined such that a carbon-12 atom has a mass of exactly 12 amu. You can convert from atomic mass units to kilograms (kg) by multiplying by 1.6605 x 10^-27, where the caret (^) indicates exponentiation.

Instructions

1. Identify or measure the molecules of concern in amu. If you're doing this as a homework problem, the text will tell you how many atomic mass units the sample has.

2. Multiply the amu measurement in Step 1 by 1.6605389 x 10^-27 to get the number of kilograms. This conversion number is in scientific notation. 10^-27 means that you move the decimal point in 1.6605389 to the left 27 positions. This number is derived from Avagadro's number (see "Tips").

3. Round to the number of significant figures of your Step 1 measure.

   For example, if you measure 101.1 amu, you'd round to 1.679 x 10^-25 kg so that both numbers have four significant figures.

   More explicitly, 101.1 amu x 1.6605389 x 10^-27 kg/amu = 1.011 x 100 x 1.6605389 x 10^-27 kg = 1.6605389 x (100 x 10^-27) kg = 1.679 x 10^-25 kg after rounding.

Tips & Warnings

• The conversion factor above can be derived as follows. If a molecule weighs X amu, then a mass of it that weighs X grams has 6.022x10^23 molecules in it. This last number is called Avagadro's number and is familiar to any high school chemistry student. So 1g = 6.022x10^23 amu. Therefore 1 kg = 6.022x10^26 amu. Therefore 1 amu = 1.661x10^-27 kg.

References

• "Chemistry"; Raymond Chang; 1984
• University of Illinois at Urbana-Champaign: Atomic Mass