What is the mass of ##NaCl## required to prepare 0.5 liters of a 2.5 molar solution of ##NaCl##?

73 g

The molecular weight of NaCl is 58.44 g/mol

So, one mole of NaCl weighs 58.44 g.

A 2.5 M solution is 2.5 moles per liter (Molarity is just the number of moles per liter).

Therefore, 0.5 L would contain 1.25 mol. Hence, you would need 1.25 × 58.44 g = 73 g.

As equations:

##M = "moles"/"vol"##

##M = "Molarity"##

##"vol" = "volume (in liters)"##

##"moles" = "grams"/(MW)##

##g = "weight of compound"##

##MW = "molecular weight"##

So,

##M = "moles"/"vol"##

Substitute for moles

##M = (g//MW)/"vol"##

We need ##g##, so rearrange

##M × "vol" = g/(MW)##

##g = M × "vol" × MW##

Put in the numbers:

##g = "2.5 mol/L × 0.5 L × 58.44 g/mol"##

##g = "73 g"##

Personally, I prefer the first approach!

You can find out more about moles and molarity here.