# How would you calculate the molecular weight of a protein?

Jul 30, 2016

You would need to find its chemical formula, then determine its molecular weight as usual. See below for an example.

#### Explanation:

Example

Hemoglobin is the protein in red blood cells that carries oxygen throughout the body. The molecular formula of hemoglobin is $\text{C"_2952"H"_4664"O"_832"N"_812"S"_8"Fe"_4}$.

Calculate its molecular weight by multiplying the subscript of each element by its atomic weight on the periodic table. **Atomic weight, and therefore molecular weight, are relative values and have no unit. However, scientists often use the unit Da for Daltons, or u for atomic mass units when writing atomic weight and molecular weight.

Calculate molecular weight of hemoglobin.

Multiply the subcripts by the atomic weights of the elements from the periodic table. The number of significant figures will be determined by the atomic masses because there is no uncertainty in the subscripts.

$\text{C} :$ $2952 \times 12.011 = 35456$
$\text{H} :$ $4664 \times 1.008 = 4701$
$\text{O} :$ $832 \times 15.999 = 13311$
$\text{N} :$ $812 \times 14007 = 11374$
$\text{S} :$ $8 \times 32.066 = 256.53$
$\text{Fe} :$ $4 \times 55.845 = 223.38$

Add the masses of each element.

$35456 + 4701 + 13311 + 11374 + 256.54 + 223.38 = 65322$ (rounded to the least number of decimal places.

In this example, the molecular weight of hemoglobin is 65322, which can also be written as 65322 Da and 65322 u.

If you want the molar mass, convert the molecular weight to grams/mol directly, which would be 65322 g/mol.