What is the molar mass of a compound if 5.19*10^16 molecules of the compound weigh 6.205 microgrammes?
The molar mass of your compound is
Since molar mass is expressed as the weight of 1 mole of a compound, you must determine exactly how many moles you've got.
Since 1 mole of a substance contains exactly
Now just divide the weight you were given by the number of moles you have to get the molar mass
SIDE NOTE Notice I've converted micrograms to grams in order to get the molar mass expressed in g/mol.