Agrapefruit has a weight on earth of 4.9 newtons. what is grapefruit's mass?

The force of gravity is the force with which massively large objects such as the earth attracts another object towards itself. All objects of the earth exert a gravity that is directed towards the center of the earth. Therefore, the force of gravity of the earth is equal to the mass of the object times acceleration due to gravity.   

F = ma 
4.9N = m (9.8 m/s^2), note that N (newtons) = kg-m/s^2 
m = 0.5 kg 
The mass of the object is 0.5kg.

Sorry i dont understand the question can you rephase it?

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.