Solution
To calculate the molar mass of a compound you multiply the subscript of each element its mass (atomic weight in g/mol) (NH
4
)
2
Cr
2
O
7
When there is no written subscript, it is understood to be 1. Also when elements are enclosed in parentheses, the subscript on the inside multiplied by the number outside the parentheses.
Molar masses of the Elements
Nitrogen : 14.007 g/mol
Hydrogen : 1.00794 g/mol
Chromium : 51.9961 g/mol
Oxygen : 15.999 g/mol
Molar mass of Ammonium Dichromate
(2×14.007)+(8×1.00794)+(2×51.9961)+(7×15.999)=252.09 g/mol
The molar mass of ammonium dichromate is 252.09 g/mol