0.496 g of NaOH required
3 pellets.
First, determine the molar mass of NaOH.
Atomic weight sodium = 22.989769
Atomic weight hydrogen = 22.989769
Atomic weight oxygen = 15.999
Molar mass NaOH = 22.989769 + 22.989769 + 15.999 = 61.978538 g/mol
Now determine how many moles of NaOH we need. Multiply desired volume by desired molarity. So
0.200 l * 0.040 mol/l= 0.008 mol
To get the mass, then multiply by the molar mass, so
0.008 mol * 61.978538 g/mol = 0.495828304 g
Since we have the NaOH in pellet form, divide the desired mass by the mass of each pellet. So
0.495828304 g / 0.170 g/pellet = 2.916637082 pellets
You'll need 3 pellets.
To get closer to the desired molarity, I would recommend increasing the amount of solution being prepared if you can accurately measure the larger volume.
