What is the percent by mass of carbon

The mass percent of a molecule shows the amount each element contributes to the total molecular mass. So, acetamide is 40.7% Carbon, 23.7% Nitrogen, 27.1% Oxygen, and 8.5% Hydrogen.

Explanation:

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8