What is the formula weight (molar mass) of a compound if 20 g of the compound in 500 mL of solution gives a 0.5 M solution?

The molar mass of a compound is calculated as below

find the number of moles

moles = molarity x volume/1000

=500 ml x0.5 / 1000 = 0.25 moles

molar mass is therefore = mass/number of moles

= 20/0.25 = 80 g/mol