How many grams of naoh are needed to make 750 ml of a 2.5 (w/v) solution?

Concentration: 2.5% w/v

Concentration w/v % = [mass of solute / volume of solution ] * 100

=> mass of solute = concentration w/v * volume of solution / 100

=> mass of solute = 2.5 * 750 ml / 100 = 18.75 grams

Also, using proportions you get to the same result:

2.5% w/v = > 2.5 grams of NaOH / 100 ml of water

=> 2.5 grams of NaOH / 100 ml water = x grams of NaOH / 750 ml water

=> x grams of NaOH = 750 ml water * 2.5 grams of NaOH / 100 ml water

=> x = 18.75 grams of NaOH

Answer = 18.75 grams of NaOH