UNIT V: lesson 6 Molar Concentration & Dilutions

C ONCENTRATION Concentration of a substance in a solution is the amount of this substance that exists in in a volume of solution. A concentrated solution: has a relatively high concentration (There is a large amount of the substance dissolved in the solution) A dilute solution: has a relatively low concentration ( very little substance is dissolved in the solution).

M OLAR CONCENTRATIONS Chemists like to use ‘mole’ to describe amount of substance in solutions. Molar Concentration or Molarity (M) is the number of moles (n) of the substance in 1 L of solution. Units for Molarity A short hand symbol for Molar Concentration is

M OLAR CONCENTRATION Ex: what is the molar concentration of 1 L of solution containing 2.5 mol of NaCl? Ex: what is the [NaCl] in a solution containing 5.12 g of NaCl in 250 mL of solution?

What mass of NaOH is contained in 4 L of 0.2 M NaOH?

What is the molariy of H 2 SO 4 having a density of g/mL?

TRY Suppose you have 5.60g of KCl and you wanted to make a 2.14 M solution. What volume of solution would you create?

D ILUTION CALCULATIONS What happens when you mix two solutions together?! Mixing two solutions together will dilute the original solutions. This means, there will be new concentrations and new volumes

DILUTION CALCULATIONS Molarity/Concentration = moles(n) Volume (L) Molarity of Mixture = Total moles (n) Total volume of mixture (L)

DILUTION CALCULATIONS *Consider a concentrated solution that we add water to in order to make a diluted solution. C I = initial concentration of solution V I = initial volume of solution After we add water…. C DIL = diluted concentration (after water is added) V DIL = (total) diluted volume (after water is added)

DILUTION CALCULATIONS Recall that c = n/V...therefore n= c·V n I = moles for Initial solutions = C I x V I n DIL = moles in DIL solution = C DIL x V DIL The Total number of moles (n) for each solution that we started with does not change when we mix the two solutions together….it stays the same.

DILUTION CALCULATIONS n I = n DIL C I x V I = C DIL x V DIL or... C I V I = C F V F

A. Simple Dilutions in water Example #1: If mL of 0.800M HCl is added to mL of water, what is the resulting [HCl] in the mixture?

B. Mixing 2 Solutions with Different Concentrations Treat mixtures of two solutions as two separate “single dilutions” Example #2: If mL of M H 2 SO 4 is added to mL of M H 2 SO 4, what is the resulting [H 2 SO 4 ] in the mixture?

C. Making a Dilute Solution from a Concentrated Solution Example #3: What volume of 8.00 M H 2 O 2 is used in making up 1.5 L of 0.5 M H 2 O 2 ?

T RY : Miss Q. mixes 200mL of water with 50 mL of a sodium chloride solution having an unknown concentration. Miss Q. finds that the molarity of the sodium chloride after dilution was M. What was the molarity of the original sodium chloride solution??

S UMMARY OF D ILUTION CALCULATIONS Always, always, always use Remember for final volume (V F ) to add up the two volumes together. It is a total volume. Be careful when you are substituting the numbers in the equation.

S UMMARY OF D ILUTION CALCULATIONS Always, always, always use To find new concentration, simply rearrange into [New concentration] = [old concentration] x old volume total volume

R EMEMBER … Molarity is the same as molar concentration. It is in moles/L If you have units in moles/mL, must change it into moles/L first. Ex: 500 mL  L Divide by 1000 = 0.5 L If you have units in grams, change g  moles by using the molar mass!

H OMEWORK Page102 #78, 80, 81, 83, 84, 85,87, 90