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.

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

To calculate molarity:

mind that there are 1000 mL in 1 L & convert to L

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

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

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

V1 = 0.625 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 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.