Molar mass prozac
C17H18F3NO = 309.33 g/mol
1 mol -------------- 309.33 g
x mol -------------- 3 g
x = 3 / 309.33
x = 0.00969 mol
1 mol ------------- 6.02x10²³ molecules
0.00969 --------- y molecules
y = 0.00969 * 6.10x10²³
y = 5.83x10^21 molecules
hope this helps!.
