What is the molarity of a solution that contain 3.25 moles of NaCl in 250 mL of water? A.) 76.9m B.) 13.0 m C.) 0.813m D.) 0.013 m

Answer 1

You need to know the definition of MOLARITY -

moles per liter of solution

Those are more than just words. You have to think of it as a fraction. And the number on the bottom of the fraction MUST be a 1. One liter of solution. When you have ONE liter of solution on the bottom, the top number is the molarity, abbreivated M.

I don't have an equation editor here - so bear with me:

# moles

_______ = M, molarity

1 L soln

So if I had 3 moles in one liter of solution, or 3 moles per liter, I would have a 3 molar, or 3M, solution.

Now that you understand the definition and the fraction,let's see what you have (I am trying to write a fraction --->)

3.25 moles (the numerator)

_____________________

0.25 L (don't forget the units must match, so this denominator MUST be converted to liters!)

Now simply use it in a ratio:

3.25/0.25 = x/1

tops of the fractions are MOLES. Bottoms of the fractions are LITERS. When Liters = 1, the x is the MOLARITY, by definition.