Question

The molar mass of calcium is 40g Why is it expressed in the unit g what is its atomic mass

Class 9

Answer 1

The atomic mass is 40

Answer 2

This is because most compounds whose molar masses we refer to every day have their molar masses in the range of a few grams to a few hundred grams, for example molar mass of anhydrous Calcium Carbonate is approximately 100 grams and that of water is 18 grams. Thus would be inconvenient to express these quantities in kilograms.

Hence the summarized reasons:
1. 1 mole is equivalent to 6.022×10^23  which is also the SI value for 1 mole. For most compounds this amount of atoms finds its weight being expressed in whole numbers in units of grams.
2. Whole numbers always easier to remember than decimal expressions.

But for very big molecules like a protein molecule its molar mass may be expressed in Kg if required.

Edit :

the SI unit for molar mass is Kg/ mole. But for historical convention molar mass is expressed in grams per mole.

The atomic mass of calcium is 40.078 g