 # Sec. 14.2: Solution Concentration

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Sec. 14.2: Solution Concentration
Ch. 14: Solutions Sec. 14.2: Solution Concentration

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. Qualitative: concentrated or dilute. 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? 3.5 %

% by volume = volume of solute x 100
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? 2.3 %

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 To calculate molarity:
mind that there are 1000 mL in 1 L & convert to L

Molarity Practice Problems
A 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 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? .28M

Preparing Molar Solutions

Preparing Molar Solutions
So, for 1 L of a 1M solution, you would need 1 mole of the salt, or g NaCl. What if you needed 1 L of a 0.3 M solution? M = moles solute or M = x moles liters solution 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 M = x moles liters solution L x = 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? 9.0 ^ -4 g CaCl2

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.

Mstock x volume stock = Mdil x volume of dilute
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

(12.0 M) V1 = (1.50 M)(5.00 L) V1 = L = 625 mL

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 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 L of a 1.25 M KI solution? 75 mL

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 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? 0.77 mol/kg

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 g H2O. 0.206