Concentrated Solution To save time and space, usually buy solutions that are concentrated- stock solutions Add solvent to get the molarity you want Dilution- process of adding more solvent to a solution
Dilutions Only water is added to the solutions Figure out how much water needs to be added Moles of solute after dilution = moles of solute before dilution
What happens? Moles of solute stays the same Volume of water increases So Molarity decreases
Example Prepare 500. mL of 1.00M acetic acetic acid, HC 2 H 3 O 2 from a 17.5 M stock solution of acetic acid. What volume of stock solution is required? 1 st - find the number of moles of acetic acid needed in the final solution V dilute x M dilute = moles solute
Example (continued) Only source of acetic acid is the stock solution So moles of solute in dilute solution must equal moles in concentrated solution This is always true!
Example (continued) Now we need to find the volume of 17.5 M acetic acid that contains 0.500 mol of HC 2 H 3 O 2 This is the unknown volume V
Volume x molarity = moles Solving for V gives V = 0.0286 L or 28.6 mL of solution To make 500 mL of 1.00M acetic acid solution, we take 28.6 mL of 17.5 M acetic acid and dilute it to a total volume of 500 mL
Because moles of solute remain the same before and after dilution, M 1 x V 1 = moles of solute = M 2 x V 2 1 is initial conditions, 2 is final conditions