# Question 24a9d

Dec 21, 2017

Needless to say, they are different. The molecular weight is the sum of the atomic weight.

#### Explanation:

[What is the atomic mass?]
The atomic mass is the average mass of an element. It is the relative mass compared to ""^12C.

Example:
""^12C=12 amu (by definition)
""^13C=13.003355 amu
(http://www.sisweb.com/referenc/source/exactmas.htm)

The abundance(%) for ""^12C and ""^13C are $98.90$ and $1.10$, respectively.
Therefore, the atomic mass of carbon is
$12 \cdot 0.9890 + 13.00 \cdot 0.0110 = 12.011$.

[What is the molecular mass?]
The molecular mass is the average relative mass of an molecule, compared to ""^12C# atom. It is the sum of the atomic mass in the molecule.

For example, the atomic mass of hydrogen is $1.008$, and the molecular mass of hydrogen(${H}_{2}$) is $2 \cdot 1.008 = 2.016$.
The molecular mass of methane($C {H}_{4}$) will be $12.011 + 4 \cdot 1.008 = 16.043$.