If Na=23, O=16 and H=1, then formula mass of NaOH= 23+16+1=40 amu. 1 mole will have a mass, called the molar mass of 40g.
What mass of NaOH is needed to make 200mL of 0.1M solution? You find the number of moles from the equation:- number of moles = volume in L x molarity = 0.200 x 0.1 = 0.020 moles.
If 1 mole has a mass of 40g, then you need to weigh out 0.020 x 40 = 0.800g