The molar mass of silver nitrate (AgNO3) is found by adding up the weight of its parts. The molecular weight of silver is 107.87, nitrogen is 14, and oxygen is 16. The sum of 107.87 + 14 + 48 (3 oxygens) equals 169.87 grams per mole.
The atomic weight of each element is not a whole number, even though protons and neutrons are each assigned a value of 1 atomic mass unit. This is because the elements have multiple isotopes, meaning the number of neutrons in a given element's nucleus varies.
In the case of silver, its atoms come in two varieties: silver-107 and silver-109. Approximately 51 percent of the silver atoms found on Earth are Ag-107, whereas the remainder are Ag-109. Averaging the relative abundance of these isotopes gives the value for silver's atomic weight found on the periodic table: 107.87 grams per mole.