Solubility, Dilution, Molarity, Molality

Solutions Our next topic in chemistry is solutions, which are a type of mixture in which a solute has been dissolved by a solvent. Today we are going to explore how to interpret and construct solubility curves. We will learn what it means for a solution to be unsaturated, saturated, and supersaturated.

Solutions Solutions are composed of a SOLVENT and at least one SOLUTE. For salt water, salt is the solute and water is the solvent. SOLUTE = substance that dissolves SOLVENT = substance into which the solute dissolves Oceans are a SOLUTION of salt plus water DEMONSTRATION: Salt in water

Solubility We learned that a solute can be dissolved in a solvent. How MUCH solute can be dissolved depends on the temperature of the solution. At some point no more solute can be dissolved, and the solution has reached saturation. The maximum quantity of a substance that will dissolve in a certain quantity of water at a particular temperature is called its solubility. We express this as “grams of [solute] per 100 g of water” for a specified temperature. A graph of a substance’s solubility at temperatures from 0°C to 100°C is called its solubility curve.

A Sample Solubility Curve 1.Do all the substances have similar curves? 2.If not, what are some differences? 3.Do all the substances’ solubilities increase as temperature increases?

Degrees of Saturation ON the curve represents saturation, the maximum amount of dissolved solute at a particular temperature. BELOW the curve the solution is unsaturated because it holds less solute than it could. ABOVE the curve the solution is supersaturated because it holds more solute than it ‘should’ for the conditions (an occasional result of letting it cool undisturbed). Disturbing such a supersaturated solution or dropping in a small ‘seed’ crystal causes rapid formation of crystals by the excess solute, leaving a saturated solution.

Solutions All SOLUTIONS are homogeneous mixtures of two or more substances.

Solutions Solute in solventExample Gas in gasAtmosphere is a mixture of mostly nitrogen and oxygen Liquid in liquidWine is mostly a mixture of ethanol and water Solid in solidAlloys such as brass are a mixture of metals Gas in liquidCarbonated soft drinks, CO 2 in sugar water Solid in liquidSeawater, dissolved salt in water Solid in gasSmoke, tiny solid particles in atmospheric gases

Steps in Solution Formation 1)Solute particles become separated from other solute particles (the solid). (Energy is absorbed). This is dissociation. 2)Solvent particles become separated from other solvent particles (moved apart to allow solute particles to enter the liquid). (Energy is absorbed) 3)Solvent particles are attracted to and surround solute particles (energy released)— the solute particles are said to be solvated.

Dissociate? Solvate? What is the difference between dissociation and solvation? Dissociation is the kinetic process of the ions in a crystal lattice of a solute becoming separated from each other (requires energy). Solute comes apart. Solvation is the process of attraction and association of solvent molecules with the dissociated ions or molecules of a solute (energy is released). Solute and solvent combine. Animation: Dissociation of Salt

Factors affecting rate of dissolution If you have ever tried to dissolve sugar in iced tea, you know that there are factors that affect how quickly you can dissolve sugar in iced tea. List 3: 1)Agitating (stirring or shaking) the solution Brings fresh solvent in contact with solute surface 2)Surface area of solute – the more SA, the faster dissolving occurs Powders dissolve more readily than do large crystals 3)Heating the solvent Increases the KE of solvent particles which means more frequent collisions with solute particles to help disperse the solute

Factors that Affect Solubility 1)Pressure—little effect on liquids or solids in liquid solvents. However, gas solubility increases with increasing pressure. (Henry’s Law states the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid at constant temperature.) 2)Temperature—Increase in temperature decreases gas solubility, but increases solid solubility (most of the time). 3)Nature of the solute vs. solvent—polar solutes dissolve readily in polar solvents but NOT in non- polar solvents and vice-versa.

Polar vs. Nonpolar Interactions “Like dissolves like” is a useful, but rough, rule for predicting whether one substance will dissolve in another. It is based upon: Polar vs. nonpolar interactions—polar substances (have a dipole moment or positive and negative ‘ends’) and ionic compounds are generally NOT soluble in nonpolar solvents such as paint remover, kerosene, turpentine, mineral oil, gasoline, and nail polish remover. Instead, polar substances and ionic compounds are soluble in water which is why water is called the ‘universal solvent.’ Types of bonding Intermolecular forces between solute and solvent—ions and polar molecules are linked by dipole-dipole attractions while nonpolar molecules are linked to nonpolar solvents with London dispersion forces.

Electrolytes A substance that dissolves in water to give a solution that conducts electric current is called an electrolyte. Sodium chloride (NaCl) and any other soluble ionic compound is an electrolyte because it contains mobile charged particles. A substance that dissolves in water to give a solution that does NOT conduct an electric current is called a nonelectrolyte. Sucrose is an example because its dissolved molecules, although mobile, are not ions and do not carry an electric charge.

Strong vs. Weak Electrolytes Strong electrolytes completely or nearly completely dissociate (form ions) and are EXCELLENT conductors of electricity (e.g., strong acids and strong bases) Weak electrolytes only partially dissociate and are POOR conductors of electricity (e.g., weak acids and weak bases) Non-electrolytes DO NOT dissociate or form ions and therefore DO NOT conduct electricity (e.g., sugars, alcohols)

Solution Equilibrium If you add spoonful after spoonful of sugar to tea, eventually there comes a point where no more sugar will dissolve. For every combination of solute and solvent, there is a limit to the amount of solute that can dissolve. Why? Eventually the rate at which sugar molecules are leaving the solid surface of the crystal and being carried away by the water is exactly balanced by the rate that dissolved molecules of sugar are returning to and recrystallizing at the crystals of sugar. This dynamic equilibrium is called solution equilibrium and it is the physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates.

Lab Techniques to Separate Mixtures Some mixtures are easier to separate than others. How would you separate tossed salad? Trail mix? Beef stew? Vinaigrette salad dressing? Orange juice? Muddy water? Milk? Consider the following separation techniques:

Decanting Centrifuging Evaporation Sifting or sieving Filtering Magnetic attraction Chromatography Crystallization Distillation

Properties of Solutions, Colloids, and Suspensions SolutionsColloidsSuspensions HomogeneousHeterogeneous Particle size nm; can be atoms, ions or molecules Particle size nm, dispersed; can be aggregates or large molecules Particle size: over 1000 nm, suspended; can be large particles or aggregates Do NOT separate upon standing Separates upon standing; particles settle out Cannot be separated by FILTRATION Can be separated by FILTRATION Do NOT scatter lightScatter light (Tyndall effect) May scatter light, but are NOT transparent Use this table and the prior slide to answer Q33 on Solubility Curve Review

Saturated, unsaturated, supersaturated

Dilution The concentration of a solution is a measure of the amount of solute in a given amount of solvent or solution. ‘Dilute’ and ‘concentrated’ are words often used without definite meanings. ‘Dilute’ just means there is a relatively small amount of solute compared to the amount of solvent, while ‘concentrated’ means a relatively large amount of solute compared to the amount of solvent.

Molarity

Beware! Note that a 1M solution is NOT made by adding 1 mol of solute to 1 L of solvent because the resulting total volume would be MORE THAN 1L! Instead, the 1 mol of solute is dissolved in less than 1L of solvent, then enough solvent is added to the result to bring the total volume to one liter (1L).

Molality

Why have both molarity and molality? Concentrations expressed in molalities are used when studying properties of solutions related to vapor pressure and temperature changes because molality is independent of temperature. NOTE: Water is the most common solvent; if the solvent is not specified, assume water. Remember that 1 ml of water has a mass of 1.000g, so that 1L of water has a mass of kg.

Activity Write steps to solve both molarity and molality problems, unnumbered, on a sheet of paper, cut/tear into strips, mix up, and exchange with a partner to reassemble. Compare with notes. Molarity: any mass of solute convert to moles, volume of solution must be in liters Molality: any mass of solute convert to moles, mass of solvent must be in kg

Sample Molarity Problem #1

Sample Molarity Problem #2 How many grams of NaCl should be used to prepare 300 mL of a 0.20 M solution? Givens:volume of solution is 300 mL (= 0.3L) molarity (M) is 0.20 M Unknown:grams of NaCl? PLAN: Use molarity equation to (1) solve for number of moles of solute, then (2) convert moles of solute to grams of solute.

Sample Molarity Problem #2

Sample Molality Problem #1

Sample Molality Problem #2 How many grams of sucrose (C 12 H 22 O 11 ) should be added to 125 g of water to prepare a m C 12 H 22 O 11 solution? Givens:125 g water = mass of solvent m = molality of solution Unknown: mass of solute, sucrose, needed PLAN: Use formula for molality and the molar mass of sucrose (C 12 H 22 O 11 ) to find the mass of sucrose in a 1.0 m C 12 H 22 O 11 solution. Then convert to a m solution and finally use a proportion to find mass of solute in 125 g of solvent (first convert g of solvent to kg of solvent).

Sample Molality Problem #2

Dilution Problems

Dilution Example