Regents Chemistry Properties of Solutions Properties of Solutions

Regents Chemistry • Properties of Solutions

Properties of Solutions • Review - What’s a solution – a solution is a homogeneous mixture of substance in the same physical state • Most chemical reactions take place in solutions • We will learn the nature and properties of solutions and ways to express the concentration of solutions

What do Solutions Contain? • Solutions contain atoms, ions or molecules in which one substance spread uniformly throughout a second substance • Ex: Salt water

Types of Solutions • Solutions exist in all three states! • A solid may be dissolved in another solid – ex: Brass is a mixture if zinc and copper • A Metal solution is called an alloy • Air is a gaseous solution and can vary depending on the conditions – ex: amount of water vapor varies daily

Liquid Solutions • We will mostly focus on solutions containing a liquid • We identify parts of a liquid solution by how it is made • Solute - is the substance that is being dissolved, and it is the substance present in the smaller amount • Solvent - substance that dissolves the solute - most common is water

Na. Cl(s) Na+(aq) + Cl-(aq) Once the salt and water is stirred and the mixture becomes homogeneous, the dissolved particles will not settle - cannot filter! Liquid solutions are clear but may have color and light will pass through without being dispersed

Liquid Solution Summary • 1. Solutions are homogeneous mixtures • 2. Solutions are clear and do not disperse light • 3. Solutions can have a color • 4. Solutions will pass through a filter

Solubility Factors • Some things dissolve in solvents and some don’t, so. . • Solubility - is how much of a solute will dissolve in a certain amount of solvent at a certain temperature • Materials with high solubility are said to be soluble • Materials with a low solubility are said to be insoluble

Nature of Solute and Solvent • Na. Cl dissolves in water because its positively and negatively charged ions are attracted to oppositely charged ends of the polar water molecule • The attractive forces between the water molecules and sodium ions are greater than the attractive forces between the sodium and chloride ions • Same goes for the chloride ions and positive end of water molecule

Like dissolves Like • Ionic substance dissolve in ionic solvents • Nonpolar substances, such as fats, dissolve in nonpolar solvents • So fats do not dissolve in water! No strong attractive forces between water molecules and fat molecules - must be dissolved in a nonpolar solvent • Why. . because the forces are weak and they simply mix together

Table summary

Effect of Temperature • As temperature increases, most solids become more soluble in water • A few exceptions exist: – Gases react in the opposite manner – As temperature increases, the solubility of all gases in liquids decreases

Effect of Pressure • Pressure has little or no effect on the solubility of solid or liquid solutes • Pressure does affect the solubility of gases in liquids • As pressure increases, the solubility of gases in liquids increases • Ex: opening a can of soda - the pressure decreases – CO 2 is no longer as soluble at the lowered pressure and escapes as bubbles

Regents Chemistry • Solubility Graphs and saturated and unsaturated solutions

Solubility Information • Solubility information may be presented in different ways • Table G in your Reference Tables shows the relationship between grams of solute that can be dissolved at various temperatures • Table F in Reference Tables provides some general guidelines about the solubility of ionic substances

Using Table G • Shows the maximum number of grams that can be dissolved in 100 g H 2 O at specific temperatures • Most show increasing solubility as temp increases, but a few don’t – these are gaseous NH 3, HCl and SO 2 – gases decrease in solubility as temp increases

Using Table G • Any point that is below the curve of a substance is considered unsaturated • Any point that is on the curve of a substance is considered saturated • Any point that is above the curve of a substance is considered supersaturated

Saturation • Unsaturated solutions hold less solute than maximum and no solid should be present • Saturated solutions hold the max amount and any additional will simply stay as a solid • Supersaturated solutions occur when the temperature is reduced but no crystals (solid) form out of solution - any additional solute added will cause crystals to form and solution will return to saturated state

Recognizing Degree of Saturation • Because solutions are clear, it is difficult to simply look at a solution and determine whether it is saturated, unsaturated or supersaturated – So how can we tell? • 1. We can compare the number of grams dissolved in a given volume to table G • 2. Add additional solute and see what happens!

Using Table F • Contains some guidelines for the solubility of common ionic compounds • YOU HAVE USED THIS TABLE BEFORE! • Explains if a reaction will form

Table G Practice Problem • Which substance on table G (solubility curve) is saturated with 20 g at 49 C? • How many grams of HCl would have to be added to a 70 g in solution to make it saturated at 10 C?

Regents Chemistry n Concentrations of Solutions – Molarity

What’s Molarity Let’s first review a mole…video clip n We sometimes refer to solutions as concentrated or dilute…but these are not scientifically precise terms. . n We need to know specific strengths to run reactions. . n – This is the purpose of molarity!

Molarity n Molarity – is the number of moles of solute in 1 Liter of solution – This tells us the exact “strength” of the solution – We add a specific amount of solute to a specific amount of water. . once this is made, the molarity doesn’t change! – The formula is below and on your reference tables Molarity = moles of solute liters of solution = g / mol L = M

Solving Basic Molarity Problems n What is the molarity of a solution that contains 4. 0 mol of Na. OH in 0. 50 L of solution? M = mole of solute liters of soln Molarity = 8. 0 M = 4. 0 moles Na. OH 0. 50 L

Molarity w / no moles given… n What is we are given a gram amount instead of mole amount…can we still solve for molarity? – Yes! Practice Problem What is the molarity of a solution containing 82. 0 g of Ca(NO 3)3 in 2. 0 L of solution? 1. Convert 82. 0 grams to moles by using molar mass 2. Plug into Molarity equation and solve!

Additional Practice Problem n What is the molarity of a solution containing 26. 0 g KCl in 750 m. L of solution?

Rearranging the Equation We can rearrange the equation to solve for mole amount or liters of solution Example n How many moles of Ba. SO 4 are in a 2. 0 M solution originally made with 1. 5 L of solution? n

Regents Chemistry n % by mass, % by volume and ppm

Percent by Mass n Common to find labels that list the concentration of ingredients by mass n Percent Mass – is simply the mss of an ingredient divided by the total mass expressed as a percent n Percent mass is essentially the same as percent composition – you have done this in lab!

Percent by Mass Percent mass= mass of part X 100% mass of whole What is the percent mass of sodium hyd If 2. 50 g of Na. OH are added to 50. 00 g o

Percent by Volume When two liquids are mixed to form a solution, it is common to express the concentration of the solute as a percent by volume n For example, a solution of isopropyl alcohol contains 70% alcohol by volume n Percent by volume = Volume of solute X 100% Volume of solution

Practice Problem n What is the percent by volume of alcohol if 50. 0 m. L of ethanol is dilluted with water to form a total volume of 300 m. L?

Parts per Million (ppm) n Parts per million is similar to % comp because it compares masses – It’s a ratio between mass of the solute to total mass of the solution n This method of reporting concentrations is useful for extremely dilute solutions when molarity and % mass would be to difficult to interpret

ppm n For example – Chlorine is used as a disinfectant in swimming pools. Only about 2 g of chlorine per 1, 000 g of swimming pool water is necessary to keep the pool sanitized Grams of solute x 1, 000 ppm = Grams of solution

Practice Problem Approximately 0. 0043 g of oxygen can be dissolved in 100 m. L of water at 20 degrees Celsius. Express this in terms of ppm n (assume 1 m. L water = 1. 0 g water n

Regents Chemistry • Colligative Properties

What are Colligative Properties? • Colligative properties are properties of a substance that are affected by the nature of a solute added to it • In terms of water: – Freezing and boiling points are colligative properties that are affected by the nature of the solute. . as we shall see…

Molecular vs. Ionic • Molecular substances affect the freezing and boiling points of water different than ionic substances. . • Why? ? • Because ionic substance break apart into ions and molecules do not! – Ex: Salt vs. sugar

Salt vs. Sugar C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) Vs. Na. Cl (s) Na+ (aq) + Cl- (aq) 1 mole of salt will raise the boiling point and depress the freezing point twice as well as 1 mole of sugar!

Vapor Pressure and Boiling Point • When a substance that is normally a liquid enters a vapor phase, it is called a vapor • A liquid normally has molecules that escape its surface • The pressure that these molecules exert in the surrounding atmosphere is called vapor pressure

Vapor Pressure • Why do these molecules escape? • Liquids are held together by rather weak intermolecular forces • These forces are called dipole-dipole forces • As temperature increases, these forces become less effective and more molecules escape…thus VP increases!

Water is different… • Water is different than most liquids. . • It participates in hydrogen bonding in addition to dipole-dipole interactions. . • Thus it has a high boiling point and requires more energy to break the intermolecular forces. . • This is seen by observing the relationship between molecular weights and vapor pressure

Table H • Table H on your reference tables shows us the vapor pressure at various temperatures. . • Notice the boiling point for each liquid • Boiling Point – is when the vapor pressure of a liquid is equal to the atmospheric pressure… • This occurs when we see bubbles!

Using Table H • Find the Vapor Pressure of water at 75 degrees Celsius. • Which of the substances has the weakest intermolecular forces? Why? • Which has the strongest intermolecular forces? Why?