Question

How many grams of sodium chloride do you need to prepare 250 mL of a 5.00%(w/v) sodium chloride solution

Answer 1

5% NaCl w/v means 5 g NaCl/100 mL soln.
So for 250 mL soln you will need
5g x (250/100) = ??

You can look at it another way.
(grams/mL)*100 = %
(X/250)*100 = 5
Solve for X.