ch. 14: solutions sec. 14.2: solution concentration
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Objectives
• Describe the concentration of solutions using different units.
• Determine the concentrations of solutions.
• Calculate the molarity of a solution.
Solution Concentration• The concentration of a solution is a measure of
how much solute is dissolved in a specific amount of solvent or solution.
dilute concentrated
Expressing Concentration
• Qualitative Descriptions– concentrated: a large amount of solute– dilute: a small amount of solute
• Quantitative Descriptions– Percent by either mass or volume– Molarity– Molality– Mole Fraction
Percent by Mass% by mass = mass of solute x 100
mass of solution Note: mass of solution = mass solute + mass solvent
• In order to maintain a sodium chloride (NaCl) concentration similar to ocean water, an aquarium must contain 3.6 g NaCl per 100 g of water. What is the % by mass of NaCl in the solution?
• The % by mass of a NaOCl solution is 3.62%. If you have 1500 g of solution, how much NaOCl do you have?
Percent by Volume
% by volume = volume of solute x 100 volume of solution Note: vol. of solution = vol. solute + vol. solvent
• What is the percent by volume of ethanol in a solution that contains 35 mL of ethanol dissolved in 115 mL of water?
• If you have 15 mL of 70% isopropyl alcohol solution, how many mLs of alcohol are in the solution?
Molarity
• Molarity (M) or molar concentration is the number of moles of solute dissolved per liter of solution.
M is read as molar. For example, 0.5 M HCl is a 0.5 molar solution of HCl. It contains 0.5 moles of HCl in every liter of solution.
Molarity Practice Problems
• A 100.5 mL intravenous (IV) solution contains 5.10 g of glucose (C6H12O6). What is the
molarity of this solution? The molar mass of glucose is 180 g/mol.
• Calculate the molarity of 1.60 L of a solution containing 1.55 g of dissolved KBr.
• What is the molarity of an aqueous solution containing 40.0 g of glucose in 1.5 L of solution?
Preparing Molar Solutions
• So, for 1 L of a 1M solution, you would need 1 mole of the salt, or 58.5 g NaCl.
• What if you needed 1 L of a 0.3 M solution?M = moles solute or 0.3 M = x moles liters solution 1 L
x = 0.3 mol or 17.6 g
• What if you only needed 150 mL of the 0.3 M solution?M = moles solute or 0.3 M = x moles liters solution 0.150 L
x = 0.045 mol or 2.63 g
Preparing Molar Solutions
• 1.0 L of a 0.10 M solution of CaCl2 is needed. How many grams of CaCl2 must be added to water to prepare this solution?
• How many grams of NaOH are needed to make 250 mL of a 3.0 M NaOH solution?
• 500 mL of a 2 M NaOH solution contains how many grams of NaOH?
Diluting Solutions
• Concentrated solutions called stock solutions are sold for laboratory use.
• Stock solutions are then diluted to prepare less concentrated solutions.
• When you add solvent to small amounts of concentrated solutions, you increase the number of solvent particles and, thus, decrease the solution’s concentration.
Diluting Solutions• Recall: M = moles solute/liters solution
• moles solute = M x liters solution
• The # of moles of solute does not change during dilution - the # of solvent particles changes.
• That means, moles of solute in stock = moles of solute in diluted solution
Mstock x volume stock = Mdil x volume of dilute
or
M1V1 = M2V2
Diluting Sol’ns Practice Problems
• What volume, in milliliters of 2.00 M of calcium chloride (CaCl2) stock solution would you use to make a 0.50 L of 0.300 M calcium chloride solution?
• If you dilute 20.0 mL of a 3.5 M solution to make 100 mL of solution, what is the molarity of the dilute solution?
• What volume of a 3.00 M KI stock solution would you use to make 0.300 L of a 1.25 M KI solution?
Molality
• If there is a temperature change, the volume of a solution will change.
• If volume changes, the molarity will change.• Therefore, there is a need for an alternative way
to express concentration.• Since masses do NOT change with temperature,
concentration can be expressed in terms of moles of solute in a mass of solvent.
Molality• The molality of a solution, denoted m, is defined as the number of moles of solute dissolved in
one kilogram of solvent:
• “1 m” is read as a 1 molal solution.• Recall: 1000 g = 1 kg
Molality• In the lab, a student adds 4.5 g of sodium
chloride (NaCl) to 100 g of water. Calculate the molality of the solution.
• A solution has naphthalene (C10H8) dissolved in 500 g of toluene. The solution has a molality of 0.468 m. How many grams of naphthalene are in the solution?
• What is the molality of a solution containing 10.0 g of Na2SO4 dissolved in 1000 g of water?
Mole Fraction
• Mole fraction is the ratio of the number of moles of solute (or solvent) in a solution to the total number of moles of solute and solvent.
• If X represents mole fraction and A represents the solute & B represents the solvent,
XA = nA XB = nB nA+ nB nA+ nB
Note: XA + XB = 1 (The sum of all the fractional components equals “the whole.”)
Mole Fraction• What is the mole fraction of HCl in a 100 g of
an aqueous solution if it contains 37.5 g HCl? What is the mole fraction of water?
• What is the mole fraction of NaOH in an aqueous solution that contains 22.8 % NaOH by mass?
• Calculate the mole fraction of NaCl in a solution in which 15.7 g NaCl is dissolved in 100.0 g H2O.