L2 - Concentration a..

Concentrations of SolutionsEquipment to LearnConcentration• The concentration of asolution describes theamount of solute that isdissolved in a givenquantity of solvent.• Two solutions cancontain the samecompounds but be quitedifferent because theproportions of thosecompounds are different.Concentrated vs. DiluteConcentrated• relatively large amount of solute per unit ofsolventDilute• relatively small amount of solute per unit ofsolventTerms are NOTinterchangeablewith strong andweak solutionsCommon Expressions ofConcentration• Molarity (M)• Molality (m)• Percent by Volume (v/v)• Percent by Mass (m/m)• Normality (N)• Parts per Trillion (ppt)• Parts per Million (ppm)• Parts per Billion (ppb)Not studiedin this textMolarity• Molarity (M) is one way to measure theconcentration of a solution.– Uses the number of solute particle in the unitsof moles and the volume of solution in theunits of litersMolarity (M) =moles of solutevolume of solution in liters– Notice that the volume involved is the totalvolume of the resulting solution, not thevolume of the solvent alone.1

So, the solution that is 0.15 M reads as a solutionthat contains 0.15 moles of sodium chloride per 1.0L of the solution. How many moles of NaCl wouldyou had 2.0 L of this solution?Your Turn:Finding Amount of Solute Given aKnown Solution Concentration• Sometimes, you may need to determinethe number of moles used in a reactiongiven the concentration of the solution.Example:Household laundry bleach is a diluteaqueous solution of sodium hypochlorite(NaClO). How many moles of solute arepresent in 1.5 L of 0.70 M NaClO.Preparing Solutions• Weigh out a solidsolute and dissolvein a given quantityof solvent.• Must add solutebefore filling tovolume1.0 L ofwater wasused tomake 1.0 Lof solution.Notice thewater leftover.2

Example:Explain how to prepare 100 mL of a 0.150M potassium permanganate solution.Not all solutes come as solidsthat can be massed. Somecompounds, along with most acids,come as concentrated solutions thatmust be diluted.Dilution is the process of addingwater to a more concentratedsolution to make a solution of lowerconcentration.Dilution = the procedure for preparing a lessconcentrated solution from a more concentratedsolution.Moles of solutebefore dilution (i)M i V iDilutionAdd Solvent==Moles of soluteafter dilution (f)M f V fDilutionDilutionSimple ratio: M 1 V 1 = M 2 V 2“watering down”Example:How many milliliters of aqueous 2.00 MMgSO 4 solution must be diluted with water toprepare 100.0 mL of aqueous 0.400 M MgSO 4 ?3

Practice Problems:1.Prepare 250 mL of 0.10 M HCl fromconcentrated HCl (12 M).2.A student needs 125g Cl - . Whatvolume of concentrated hydrochloricacid is needed to contain therequired chlorine?Molality (m)Because both moles and mass donot change with temperature,molality (unlike molarity) is nottemperature dependent.m =mol of solutekg of solventPractice Problem:1. A solution is prepared by mixing1.00 g ethanol with 100.0 g water togive a final volume of 101 mL.Calculate the molarity and molalityof the solution.4