Ch+8+Reading+Guide

__**Reading Guide for Ch 8: Solutions**__ The main point of Chapter 8, Solutions, is that solutions are very common in nature, they are essential to the process of life, and being able to understand their compositions and properties is a necessary part of Chemistry and medicine.

__8.1: Solutions__ ->Summary: Solutions are mixtures that contain a solute (relatively smaller amount of substance) that is dissolved in a solvent (relatively larger amount of substance) while maintaining the properties of both substance. Water is a very common solvent because of its polarity and hydrogen bonding. Dissociation into ions happens because the more positive pole of the polar solvent attracts the negative ion of a salt while the more negative pole of the solvent attracts the positive ion. This explains why polar and ionic substances dissolve in other polar substances.

->Struggling Topic 1: Is there a rule for identifying the solute and solvent in solution when the amounts of each substance are unknown or in different units? For example if I know that 10 g of A is mixed with 20 mL of B, how could I identify the solute and the solvent? Normally the solute is the solid and the solvent is the liquid. In case you have two liquids or two gasses, then the solute is the one in the lowest amount and the solvent is the one with the highest amount.

->Struggling Topic 2: Is there an easy way to look at a compound and identify if it is polar or nonpolar? students struggle to identify polar and nonpolar compounds. This makes understanding and applying "like dissolves like" harder. It all have to do with eletronegativity. In general, the only two elements with the "same" electronegativity are carbon and hydrogen, so compounds that only have these two elements are non polar. The tricky part of this is that compounds that have other elements but they have a symetric 3D structure are also non-polar.

->Struggling Topic 3: Why do nonpolar solutes dissolve in nonpolar solvents? They have the same intermolecular interactions. They have the "same personality" so they can interact with each other without undergoing in repulsions.

__8.2: Electrolytes and Nonelectrolytes__ -> Summary: Strong electrolytes dissociate completely into ions, weak electrolytes dissociate partially into ions, and nonelectroyltes do not dissociate at all. Equivalencies are measurements of amounts of ions that are needed to produce positive or negative 1 charge.

->Struggling Topic 1: From their chemical formulas, is there a way to determine if an electroylte will be strong or weak? I know that ionic bonds typically make stronger electrolytes than covalent bonds, but some like HF change the trend. I think HF is a weak electrolyte because the F ion is so electronegative that the hydrogen bonds between water molecules have little effect on them. The HF case also have to do with acid strenght and wie will cover this later in the semester.

->Struggling Topic 2: Equivalents are used to easily compare positive and negative charges in solutions. Why do both positive and negative charges receive only positive equivalencies? Is this because an equivalency is a measurement of how many ions are present and you cannot have a negative number of ions?

__8.3: Solubility__ -> Summary: Solubility is a measure of how much solute will dissolve in a given solvent at specific temperature and pressure; if the solution cannot hold more solute, then it is saturated. If more solute could dissolve, then the solution is unsaturated. Temperature and pressure affect the solubility of a substance according to thermodynamic laws and henry's law. Some salts are soluble and some are insoluble based upon the ions from which they are composed, and sometimes as solutions mix, an insoluble salt can form and come out of solution to become a solid precipitate.

-> Struggling Topic 1: Students often struggle to understand why (assuming constant pressure) temperature is directly related to the solubility of a solid-liquid solution but indirectly related to the solubility of a gas-liquid solution. The best way I can explain it is the same way our book did: an increase in temperature increases the movement/energy of molecules, and when the liquid's molecules move faster the solid dissolves more easily, but when the gas' molecules move faster, they are more likely to move out of solution. Is there an easier or simpler way to explain this?

-> Struggling Topic 2: The general rule for salt solubility in water is that if they include at least one of the following ions: lithium, sodium, potassium, ammonium, nitrate, or acetate, and many chlorides then they will be water soluble. Is it common to expect students to know this without consulting a table or reference page? If so, is there another way to grasp this rule?

__8.4:Percent Concentration__ -> Summary: Concentration describes how much solute is present per amount solvent. This can be expressed as a percent between the mass of solute and the whole solution, as a percent between the volume of solute and the whole solution, and as a percent between the mass of solute and the volume of the whole solution. These concentration percents may also be used as conversion factors to determine how much solute, solvent, or solution is present given any of the other information.

->Struggling Topic 1: How do you find the mass/volume percent of a solution if you know only the mass of the solute and the volume of the solvent? Does this mean that you always will make a mass/volume solution by first measuring the solute properly and then adding solvent until you have the needed volume? I sometimes mess these problems up because I forget that the solute in solution still contributes to the total solution's volume.

->Struggling Topic 2: Are mass/volume percents directly related to molarity? If molar mass can be used to relate grams and moles, then it seems reasonable to be able to relate moles or grams to the volume of the solution. Is there a preferred method for measuring concentration? What different applications do these have?

__8.5: Molarity and Dilution__ -> Summary: Molarity is a concentration found by dividing moles of solute by liters of solution, and this may be used as a conversion factor between moles of solute and liters of solution. Dilution is the process of adding water to a solution to increase the solution's volume and thus decrease its molarity.

-> Struggling Topic 1: Is molarity directly related to mass/volume percents? If moles of a substance can be converted into grams using molar mass, then it seems like we could easily relate mass/volume percents to molarity. Are there times when one concentration measurement is preferred over others? It depends on the field. What varied applications do these measurements usually have?

-> Struggling Topic 2: Why does the author teach M1V1 = M2V2 if the students are used to using conversion factors? I think it would be better to simply have the students set up molarity as a number of moles divided by 1 L. Then it is clear that you must multiply by volume to find the number of moles. This should help students avo id a mix up between 1000 mL and 1 L when they do conversions.

__8.6: Properties of Solutions__ -> Summary: Semipermeable membranes allow only water and small solute particles to pass through them. Colloids are homogeneous mixtures that do not separate or settle because colloid particles are small enough to pass through filters but too large to pass through membranes. Suspensions are heterogeneous mixtures that are so large that they are trapped by filters and membranes and can often be seen with the naked eye. Osmosis is the movement of water from an area of lower solute concentration to an area of higher solute concentration. Osmotic pressure is greatest in solutions with the highest solute concentration. Isotonic (physiological) solutions exert the same osmotic pressure as body fluids. Hemolysis occurs when fluid flows by osmosis into a cell that increases in volume and may burst. Crenation occurs when fluid flows by osmosis out of a cell that decreases in volume and shrinks. Dialysis uses osmosis and diffusion to separate small solute molecules and ions from larger colloid and protein molecules.

-> Struggling Topic 1: I used to teach my students that in osmosis water flowed from the higher concentration of water to the lower concentration of water (downhill). This may be true, but it is a confusing way to look at concentration. Most of the time water is the solvent in solution, and in these cases, concentration would refer to the solute that is dissolved in water NOT to the water itself. Plus, it is perhaps simpler to teach that water flows towards the greater concentration of solute anyway. Am I correct to assume that talking about the concentration of a solvent is at best unusual and possibly incorrect? You assumption works, and if you are teaching the concept to non-science major this water concentration idea works. However, if you are teaching this concept to science major it is better to use concentration in terms of solute, because the concentration of solvent in solution is always constant, and this have applications in equilibrium problems etc.

-> Struggling Topic 2: Identifying crenation and hemolysis requires knowledge of the isotonic solution (.9 % m/v NaCl and 5% m/v glucose). Hypothetically, could you reach equilibrium by having a crenation NaCl situation happen simultaneously to a hemolysis glucose solution? If a cell were placed in a 2% m/v NaCl solution and a 3% m/v glucose solution could equilibrium be maintained by the water trying to leave the cell because of the NaCl solution but trying to enter the cell because of the glucose solution? No, when we are talking about the solution, the solution will consist of both the salt and the sugar, so if the ratio of water to solute (salt and sugar) is low then you have an excess of water thus making the solution hypertonic, so crenation will happen.

__Critiques of this chapter__ --A. How clearly the author communicated individual topics ---> 8.1 - it was unclear how to determine which substance is the solute and which is the solvent in a solution. The author mentioned that the solute was usually the substance with the smallest amount present, but there wasn't a rule to follow. --->8.3 - there was great diagram/picture about pressure and solubility (Henry's Law) on p.300 that shows the difference between a can of soda before it is opened and as it is opened. This clearly shows how the pressure of gas outside the soda affects the amount of gas dissolved in the soda. --->8.6 - the pictures of solution, colloid, and suspension particles showing the particles' behaviors (filtering, settling, and passing through a membrane) made a clear representation of the differences between these particles.

--B. Specifics about the amount of content (did you need more examples?) --->8.5 - the author solved molarity conversion factor problems in two different ways. This was good to help students get a feel for their preferred method for doing the problems. It might have been more useful to give an extra example problem for each method of solving these problems, however.

--C. Chapter's place in the overall text ---> 8.3 - it was clear that the knowledge of ions and polyatomic ions that are found in salts was needed before discussing solubility. This explains why polyatomic ions were discussed in Ch 4 and solubility in Ch 8. --->8.5 - it is good that concentration conversion factors come after more simple conversion factors from previous chapters. It should be easier to relate molarity and m/m or v/v percents as conversion factors once students are familiar with using Avagadro's number, molar mass, and other conversion factors.