Molarity, Dilution, and pH. Main Idea: Solution concentrations are measured in molarity . Dilution is a useful technique for creating a new solution from a stock solution. pH is a measure of the concentration of hydronium ions in a solution. Properties of Aqueous Solutions.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Molarity, Dilution, and pH Main Idea: Solution concentrations are measured in molarity. Dilution is a useful technique for creating a new solution from a stock solution. pH is a measure of the concentration of hydronium ions in a solution.
Properties of Aqueous Solutions • Solution-a homogeneous mixture of two or more substances. • Solute-a substance in a solution that is present in the smallest amount. • Solvent-a substance in a solution that is present in the largest amount. • In an aqueous solution, the solute is a liquid or solid and the solvent is always water.
Molarity Review • One of the most common units of solution concentration is molarity. • Molarity (M) is the number of moles of solute per liter of solution. • Molarity is also known as molar concentration, and the unit M is read as “molar.” • A liter of solution containing 1 mol of solute is a 1M solution, which is read as a “one-molar” solution. • A liter of solution containing 0.1 mol of solute is a 0.1 M solution.
Molarity Equation • To calculate a solution’s molarity, you must know the volume of the solution in liters and the amount of dissolved solute in moles. • Molarity (M) = moles of solute liters of solution
Molarity Example A 100.5-mL intravenous (IV) solution contains 5.10 g of glucose (C6H12O6). What is the molarity of the solution? The molar mass of glucose is 180.16 g/mol. SOLUTION: • Calculate the number of moles of C6H12O6 by dividing mass over molar mass = 0.0283 mol C6H12O6 • Convert the volume of H2O to liters by dividing volume by 1000 = 0.1005 L • Solve for molarity by dividing moles by liters = 0.282 M
Preparing Molar Solutions • Now that you know how to calculate the molarity of a solution, how would you prepare one in the laboratory? • STEP 1: Calculate the mass of the solute needed using the molarity definition and accounting for the desired concentration and volume. • STEP 2: The mass of the solute is measured on a balance. • STEP 3: The solute is placed in a volumetric flask of the correct volume. • STEP 4: Distilled water is added to the flask to bring the solution level up to the calibration mark.
Properties of Aqueous Solutions • All solutes that dissolve in water fit into one of two categories: electrolyte or non-electrolyte. • Electrolyte-a substance that when dissolved in water conducts electricity • Non-electrolyte-a substance that when dissolved in water does not conduct electricity. • To have an electrolyte, ions must be present in water.
Electrolytic Properties of Aqueous Solutions • NaCl in water. • What happens? • NaCl(s)→ Na+(aq) + Cl–(aq) • Completely dissociates
Strong vs. Weak Electrolytes • How do you know when an electrolyte is strong or weak? • Take a look at how HCl dissociates in water. • HCl(s)→ H+(aq) + Cl–(aq)
Electrolytic Properties of Aqueous Solutions • What about weak electrolytes? • What makes them weak? • Ionization of acetic acid • CH3COOH(aq)↔ CH3COO–(aq) + H+(aq)
Precipitation Reactions • Precipitation Reaction-a reaction that results in the formation of an insoluble product. • These reactions usually involve ionic compounds. • Formation of PbI2: • Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
Precipitation Reactions • How do you know whether or not a precipitate will form when a compound is added to a solution? • By knowing the solubility of the solute! • Solubility-The maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. • Three levels of solubility: Soluble, slightly soluble or insoluble.
Determining Solubility • Determine the solubility for the following: (1) Ag2SO4 (2) CaCO3 (3) Na3PO4
Diluting Molar Solutions • In the laboratory, you might use concentrated solutions of standard molarities, called stock solutions. • For example, concentrated hydrochloric acid (HCl) is 12 M. • You can prepare a less-concentrated solution by diluting the stock solution with additional solvent. • Dilution is used when a specific concentration is needed and the starting material is already in the form of a solution (i.e., acids).
Dilution of Solutions • When you want to dilute a solution, what happens to the number of moles present in the solution? • Do they increase? • Decrease? • Stay the same?
PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Add water to the 3.0 M solution to lower its concentration to 0.50 M Dilute the solution!
PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? But how much water do we add?
moles of NaOH in ORIGINAL solution = moles of NaOH in FINAL solution PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? How much water is added? The important point is that --->
PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Amount of NaOH in original solution = M • V = (3.0 mol/L)(0.050 L) = 0.15 mol NaOH Amount of NaOH in final solution must also = 0.15 mol NaOH Volume of final solution = (0.15 mol NaOH) / (0.50 M) = 0.30 L or 300 mL
PROBLEM: You have 50.0 mL of 3.0 M NaOH and you want 0.50 M NaOH. What do you do? Conclusion: add 250 mL of waterto 50.0 mL of 3.0 M NaOH to make 300 mL of 0.50 M NaOH.
Preparing Solutions by Dilution A shortcut M1 • V1 = M2 • V2 Where M represents molarity and V represents volume. The 1s are for the stock solution and the 2s are for the solution you are trying to create.
Colligative Properties • Properties that depend only on the number of solute particles and not on their identity.
Some Colligative Properties are: • Vapor pressure lowering • Boiling point elevation • Freezing Point depression
Vapor Pressure Lowering • The particles of solute are surrounded by and attracted to particles of solvent. • Now the solvent particles have less kinetic energy and tend less to escape into the space above the liquid. • So the vapor pressure is less.
Ionic vs Molecular Solutes • Ionic solutes produce two or more ion particles in solution. • They affect the colligative properties proportionately more than molecular solutes (that do not ionize). • The effect is proportional to the number of particles of the solute in the solution.
How many particles do each of the following give upon solvation? • NaCl • CaCl2 • Glucose
Example • Salt is added to melt ice by reducing the freezing point of water.
Example • Addition of ethylene glycol C2H6O2 (antifreeze) to car radiators.
Freezing Point Depression and Boiling Point Elevation Boiling Point Elevation • ∆Tb =mkb (for water kb=0.51 oC/m) • Freezing Point Depression • ∆Tf=mkf (for water kf=1.86 oC/m) • Note: m is the molality of the particles, so if the solute is ionic, multiply by the #of particles it dissociates to.
Which is more effective for lowering the freezing point of water? • NaCl or CaCl2
Example 1: • Find the new freezing point of 3m NaCl in water.
Example 2: • Find the new boiling point of 3m NaCl in water.
ThepH scaleis a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion.Under 7 = acid 7 = neutralOver 7 = base
Acid-Base Reactions • Acids-generally have a sour taste, change litmus from blue to red, can react with certain metals to produce gas, conduct electricity. • Bases-generally have a bitter taste, change litmus from red to blue, feel slippery, conduct electricity. • BrØnstead Acid-proton donor • BrØnstead Base-proton acceptor
Acid-Base Reactions • Acid or Base? • HCl(aq) + H2O(l)→ H3O+(aq) + Cl–(aq) • NH3(aq) + H2O(l)→ NH4+(aq) + OH–(aq)