Solution: It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration). A 40.39 gram sample of a hydrate of Ca(NO3)2 was heated thoroughly in a porcelai

It is often possible to change a hydrate into an anhydrous compound by heating it to drive off the water (dehydration). 

A 40.39 gram sample of a hydrate of Ca(NO₃)₂ was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 28.07 grams of the anhydrous compound remained. What is the formula of the hydrate? 

Use a period instead of a dot in the formula of the hydrate.