How many moles of nacl are required to make 250 ml of a 3.00 M solution?

3.00 M, or 3 moles per (L) "liter" calls for having 3 moles per liter of the solution.

The question asks how many moles must be in 250ml of a solution that has 3 moles per Liter.

You must ask yourself what percent of 1 Liter is 250mls? Since there are a thousand ml in one liter, (1000ml=1L), then 250ml is exactly 25% of a Liter, or .25L.

So, 250ml can only hold 25% of the 3.00 Molarity. Meaning that you multiply 3 x .25 and get .75 moles.