How many millimoles and molecules of solute are in 500 ml of a solution that is 5.41 ppm KMnO4?

Answer 1

5.41 ppm = 5.41 mg/L.
5.41 mg/L x 0.5L = ?mg
?mg x (1 g/1000 mg) = ? g
mols = g/molar mass = ?
Multiply by 1000 to convert to millimols.

For number of molecules, remember that 1 mol contains 6.022E23 molecules. Use that with the number of mole from above.