Chemistry 110

Chapter Seven Notes

Chapter Seven: Solutions Solution Terms „ A solution is a mixture, composed of two parts: { The solute (or solutes), which is the component of the solution being dissolved. „ The solute need not be a solid. { The solvent, which is the medium in which the solute is dissolved. „ The solvent is present in a larger amount than the solute „ For example, sugar (a solute), can be dissolved in water (a solvent), to form a sugar-water solution. „ If the solution contains a relatively large amount of solute, we say that the solution is concentrated. „ If the solution contains a relatively small amount of solute, we say the solution is dilute. „ The key word here is relative. Solubility „ The solubility of a compound quantifies the amount of solute which can be dissolved in a given solvent. „ If a compound is soluble in a given solvent, this means that a significant amount can be dissolved. „ On the other hand, if a compound is insoluble, very little or none of it will dissolve. „ The exact solubility of the compound is often temperature dependent, with most non-gas compounds dissolving in greater amounts at higher temperatures. { Note that gases are usually less soluble at higher temperatures. „ Solubility is usually measured in grams of solute per 100. grams of solvent. „ These values can usually be read off of a graph comparing temperature with solubility. „ Example: What mass of potassium chloride can be dissolved in 2.50 kg of water at 40 °C if the solubility of KCl at this temperature is 42 g/100 g water (see graph)?

Saturation

„ As we have seen, there is a maximum amount of solute which can be dissolved in a solvent at a certain temperature.

„ If more solute has been added than the solution can dissolve, the solution is said to be saturated.

„ If this solubility limit has not been reached for a solution, it is said to be unsaturated. Solubility of Gases „ Many gases are able to dissolve in water to some extent { Consider carbon dioxide dissolved in water in soft drinks „ In general, the solubility of gases is dependent on the temperature of the solution and the pressure of the gas above the liquid { Gases are generally more soluble at lower temperatures (in contrast to most solids) { The higher the pressure of a gas above a solvent, the more of it we expect to dissolve in the solvent

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Chemistry 110

Chapter Seven Notes

Solubility Rules „ It is important to know whether or not some common chemicals are soluble or not at room temperature „ You should know the following solubility rules (in water) by memory: { All nitrates are soluble. + + + { All salts of Group I cations (Li , Na , etc.) and ammonium (NH4 ) are soluble. + 2+ 2+ { All chlorides, bromides, and iodides are soluble, except those of Ag , Pb , and Hg2 . + 2+ 2+ 2+ { All hydroxides are insoluble except those of Group I, NH4 , Ba , Sr , and Ca . „ Ca(OH)2 is only slightly soluble. Concentration „ The concentration of a solution is a measurement of how much of a solute it contains „ There are several different methods we can use to measure concentration { Molarity { Percent by Mass, Percent by Volume, and Mass-Volume Percent { Equivalents and Milliequivalents { Other methods not studied in this class Molarity „ One of the most useful terms for concentration relates the number of moles of solute in a liter of solution; it is called the molarity, and abbreviated “M”:

molarity(M) =

mol solute L solution

„ For example, to prepare a 1.0 M NaCl solution, we take 1.0 mol of NaCl (58.44 g) and add enough water to it to bring the total volume of the solution to 1.0 L.

„ Likewise, a 2.0 M NaCl solution will contain 2.0 mols of salt (2 µ 58.44 g) in every liter of the solution Example What is the total mass of KCl dissolved in 2.50 L of a 0.550 M solution of potassium chloride?

Mole-Volume Problems „ Since the molarity of a solution relates the volume to the number of moles of a solute, we can use it to help us calculate the amount of product formed in a reaction. „ First, we must convert the volume of a solution to moles of solute, using the molarity. „ From there, we can use the mole ratios of the chemical equation to figure out how much product is produced.

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Chemistry 110

Chapter Seven Notes

Examples 25.00 mL of a 0.45 M AgNO3 solution is treated with excess NaCl solution, producing silver chloride and sodium nitrate. (a)Which of these products is insoluble? (b) What mass of this product should be produced?

Mass Percent The mass percent tells us what percent of the mass is derived from the solute:

„

mass percent =

„ „

g solute g solute × 100% = × 100% g solute + g solvent g solution

The symbol %(m/m) is used to specify the fact that we are measuring percents by mass For example, a 10.0%(m/m) aqueous NaCl solution contains 10.0 g of NaCl per 90.0 g of water.

Volume Percent „ When two liquids are combined, the volume percent can be used to quantify the amount of the liquid solute:

volume percent =

„ „

volume of liquid solute × 100% volume of solution

The symbol %(v/v) is used to specify the fact that we are measuring percents by volume So, a 15%(v/v) methanol solution contains 15 mL of methanol for every 100. mL of total solution.

Mass-Volume Percent „ The mass-volume percent tells us the mass of solute (in grams) dissolved in every 100 mL of solvent

mass-volume percent = „ „

mass of solute (g) × 100% volume of solution (mL)

The symbol %(m/v) is used for this type of measurement So, a 15%(m/v) sugar solution contains 15 grams of sugar for every 100. mL of total solution.

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Chemistry 110

Chapter Seven Notes

Examples a. 45.8 g sodium nitrate is combined with 54.9 g distilled water. What is the mass percent of NaCl in the resulting solution?

b. How many grams of sodium chloride are there in 35.0 mL of a 22.0 %(m/v) NaCl solution?

Equivalents and Milliequivalents „ In the context of solutions, we use equivalents to keep track of the total charge of ions in solution „ The number of equivalents of an ion present in solution is found by multiplying the absolute charge of the ion by the number of moles of it in the solution + { A solution which contains 2.0 moles of Na ions contains 2.0 equivalents 3{ A solution which contains 2.0 moles of N ions contains 6.0 equivalents „ The concentration of ions present in bodily fluids is often measured in milliequivalents per liter of the fluid { 1 equivalent = 1,000 milliequivalent + + „ For example, a solution which contains 150. mEq/L of K ions contains 0.150 moles of K ions in a liter of the solution 2+ „ A solution which contains 200. mEq/L of Ca ions contains 0.100 moles of calcium ions per liter { Since calcium has a 2+ charge, we divided the number of equivalents by the charge to determine the moles of calcium present Example

„ A 1.00 liter IV bag contains 25.0 grams of NaCl and 10.0 grams of KCl. Determine the concentration of each ion in the IV bag in mEq/L.

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Chemistry 110

Chapter Seven Notes

Dilutions „ A common laboratory procedure is the dilution of a concentrated solution to one of a lesser molarity. „ The necessary calculations for this can be accomplished either by using dimensional analysis, or by using the relationship

C1V1 = C2 V2 where C stands for concentration, and V stands for the volume of the solution. Example „ A chemical stockroom has a 6.00 M NaOH solution on hand (called a “stock solution”), which a chemist must use to prepare 10.0 L of 0.15 M solution. What volume of the stock solution is required?

Colligative Properties „ Colligative properties describe how the presence of a solute effects the solvent. „ The specific colligative properties we will consider are { Lowering of the solvent’s vapor pressure { Boiling point elevation { Freezing point depression { Change in osmotic pressure The Effect of Solute Particles „ When solute particles are added to a solvent, they “get in the way” of the solvent’s molecules. „ The vapor pressure of a solvent may be lowered when a solute is added, as the solute particles may block the surface of the solution, making it more difficult for the solvent particles to evaporate. „ For the same reason, the boiling point is raised, as more energy is required for solvent molecules to break through the surface. Freezing Point Depression „ The freezing point of a solution is lower than that of the pure solvent. „ As the solvent molecules in a solution try to form a solid in the freezing process, the solute particles may “get in the way.” „ Extra cooling is required to solidify the solution. Osmotic Pressure „ When two solutions of unequal concentration are separated by a semi-permeable membrane (one that only allows water to flow through it), solvent will flow from the less concentrated solution, through the membrane, into the more concentrated solution to equalize the two. This process is called osmosis. „ The pressure that would have to be applied to prevent this from occurring is called the osmotic pressure. „ In general, the greater the difference in the concentration of the solutions, the greater the osmotic pressure.

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Chemistry 110

Chapter Seven Notes

Osmolarity „ The magnitude of the osmotic pressure depends largely on the number of solute particles present „ When 1 mole of NaCl dissolves, a total of 2 moles of particles (1 mole of Na+ and 1 mole of Cl-) disperses into the water „ On the other hand, if 1 mole of FeCl3 dissolves, 4 moles of particles are dispersed (one mole of Fe3+ ion and three moles of Cl- ions) „ The osmolarity of a solution is determined by multiplying the molarity (M) of a solution by the number of particles given produced by the solute when a unit of the substance dissolves „ What is the osmolarity of 0.450 M solutions of (a) NaCl and (b)BaCl2?

Osmosis and Red Blood Cells „ The structure of some cells in our bodies depends on the concentration of particles in the solution surrounding the cells „ We will consider red blood cells and their sensitivity to their environments „ Typically, the fluid in a red blood cell has concentration of NaCl = 0.9% (m/v) „ A solution with this same concentration is said to be isotonic to the cell „ Cells in isotonic solutions will take in water and expel it at essentially the same rate „ Suppose that a cell is placed in a solution with NaCl concentration 1.50% „ The concentration of ions outside of the cell is greater than that inside the cell. We say the solution is hypertonic relative to the cell. „ Water will tend to flow out of the cell more rapidly than it enters due to osmosis. „ The cell will shrink and deform its shape in a process called crenation „ Suppose that a cell is placed in a solution with a very low NaCl concentration of 0.10% „ The concentration of ions inside the cell is greater than that outside the cell. We say the solution is hypotonic relative to the cell. „ Water will tend to flow into the cell more rapidly than it leaves. „ The cell will expand and may eventually burst in a process called hemolysis

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