03 - CONCENTRATION -

DILUTIONSCHEMISTRY 30 – UNIT 2 – SOLUBILITY – CH.

16 IN TEXT

STOCK SOLUTIONS

• A common situation in a chemistry laboratory occurs when a solution of a particular concentration is desired, but only a solution of greater concentration is available: this is called a stock solution.

WHAT IS A DILUTION?

• To dilute a solution means to add more solvent without the addition of more solute.

• You may have heard it referred to as “watering down”• This statement is somewhat accurate, as long as your

solvent actually is water• Of course, the resulting solution is thoroughly mixed so

as to ensure that all parts of the solution are homogenous.

WE CAN DILUTE A SOLUTION BY ADDING WATER TO IT:

BEFORE DILUTION: M1 = INITIAL MOLES OF SOLUTE

AFTER DILUTION: M2 = FINAL MOLES OF SOLUTE

FROM PREVIOUS SLIDE…

• NOTICE THAT WE DID NOT ADD ANY MORE SOLUTE AT ALL • THE NUMBER OF MOLES WILL NOT CHANGE.

• THE VOLUME CHANGES, AND ALSO THE CONCENTRATION.

Initial moles of solute

Final moles of solute…is equal

to

DILUTION EQUATION:• The fact that the solute amount stays constant allows us to

develop calculation techniques. First, we write:

• From the definition of molarity, we know that the moles of solute equals the molarity times the volume.

• So we can substitute equation, like this:

• M1V1 means the molarity and the volume of the original solution

• M2V2 means the molarity and the volume of the second solution.

CAUTION!

• You may also see this equation written as C1V1 = C2V2. • “C” stands for concentration.

• We use the formula M1V1 = M2V2 when our concentration is expressed in molarity.

• We would use C1V1 = C2V2 if our concentration was expressed in something else, like g/L, or ppm, etc.

EXAMPLE #1 • 53.4 mL of a 1.50 M solution of NaCl is on hand, but you

need some 0.800 M solution. How many mL of 0.800 M can you make?• Using the dilution equation, we write:

• Any old volume measurement is fine, just so long as the same one is used on each side.

EXAMPLE #2

• 100.0 mL of 2.500 M KBr solution is on hand. You need 0.5500 M. What is the final volume of solution which results?• Placing the proper values into the dilution equation gives:

SERIAL DILUTIONS

• The dilution equation is fairly easy to understand – we shouldn’t have any trouble calculating new or desired concentrations or volumes• Sometimes however, we will calculate out a

value that is too small or unrealistic – this makes it difficult to measure in a practical lab-like setting

SERIAL DILUTIONS - EXAMPLE

• How could we prepare 200.0 mL of .01 M HCl from 10. M HCl?• Calculate the missing value using the dilution equation

• M1v1 = M2v2 10. ( vi ) = 0.200 (.01)• v1 = 0.00020 L or 0.20 mL

• It is unrealistic for us to measure .20 mL so we do a series of dilutions so we can maintain our accuracy

SERIAL DILUTIONS - EXAMPLE• How do we solve this?

• Step 1: First pick a practical dilution into an appropriately small volume. This is usually determined by the tools that you have in the lab.• For example use 10. ml into 200.0 ml• M1V1 = M2V2

• 10M ( 10mL ) = ( M2 ) 100mL M2 = 1.0 M• Step 2: Now you have a 1.0 molar solution so redo the math to

find how much of our NEW solution we need to make the concentration we want.• M1V1 = M2V2

• (1.0)v1 = 200.0mL ( .01 ) v1 = 2 mL• 2 mL is a much more reasonable amount to measure than 0.2 mL

SERIAL DILUTIONS - EXAMPLE

• If 2 mL is still too small and you don't have the equipment to do it, continue to dilute:• Step 1: Do the 10. ml into 100. ml again; this time your initial

concentration will be 1.0 M• M1v1 = M2v2

• (1.0) 10. = (M2) 100 M2 = .10 M• Step 2: Now you have a .10 molar solution so redo the math

• M1v1 = M2v2

• (.10)v1 = 200 (.01) v1= 20 mL• 20 mL is even easier than 2 mL to measure accurately

SERIAL DILUTION VISUAL

Stock Solution M1 = 10.M

Volumetric PipetteV1 = 10.0mL X 1st Dilution

M2 = ?MVolumetric Flask

V2 = 100.0mLM1V1=M2V2Gives us 1.0M

1st DilutionM1 = 1.0M

Graduated PipetteV1=?mL X 2nd Dilution

M2 = 0.01MVolumetric Flask

V2 = 200.0mLM1V1=M2V2Gives us 2.0mL