The number 6 X 10 ²³ is calculated so that a mole of any compound weighs as much as its molecular formula expressed in grams.

For example, the formula mass of ethane() is 30 atomic mass units. The mass of 6 X 10 ²³ ethane molecules weigh 30 grams.

The atomic masses of some elements are listed
C=12, O=16, N=14, Be=4,H=1.