How many molecules are in 4.00 x 10^2mg of ibuprofen?

Start by converting mg to g. There is .001g in every miligram, so there is 0.4g in this sample.

Then find the molar mass of ibuprofen (C13H18O2) which is 206.3g/mol

Then divide grams by the molar mass to get moles of C13H18O2: (0.4g)/(206.3g/mol) = 1.94x10^-3mol C13H18O2

Then multiply moles by Avogadro's number to get molecules: (1.94x10^-3mol)/(6.02x10^23) = 1.17x10^21 molecules of ibuprofen (C13H18O2)