Molarity

Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution:

Molarity (M) = moles solute/liters solution
For example, a 0.25 M NaOH solution (this is read as 0.25 molar) contains 0.25 moles of sodium hydroxide in every liter of solution. Anytime you see the abbreviation M you should immediately think of it as mol/L.



EXAMPLE:
What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution?

Method 1:

Calculate the number of moles of solute present.
mol NaOH = 15.0g NaOH x 1 mol NaOH/40.0 g NaOH = 0.375 mol NaOH

Calculate the number of liters of solution present.
L soln = 225 mL x 1L/ 1000mL =0.225 L soln


Divide the number of moles of solute by the number of liters of solution to get the Molarity.
M = 0.375 mol NaOH/0.225 L soln = 1.67 M NaOH












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