how many grams of NaOH are needed to prepare 250 mL of 0.205 M NaOH?

Answer 1

Well, it's 0.205 moles/liter and you're making 1/4 liter, so you need 0.205/4 or 0.05125 moles. The molar mass of NaOH is equal to the sum of the molar masses of Na, O, and H, or 22.990 + 15.999 + 1.007 = 39.996 g/mole. The mass on NaOH needed is equal to 0.05125mole*39.996 g/mole = 2.050 grams.

Answer 2

Rather than answer something that sounds like homework I'll tell you how to figure it out.

First figure out how much NaOH is needed for 1 liter of 1 M solution.

Then multiply that figure by 0.205, this converts the amount needed to make 1 liter of 0.205 M solution.

Then multiply by 250/1000 (0.250) to correct the amount to the volume you need.