chem unit 6 quest

What are solubility curves?

Solubility curves are graphs that show different solutes and the amounts of them that dissolve in 100 grams of water. Note: since the density of water is one, its mass equals its volume (so milliliters equals grams). The x-axis’ of these graphs represent temperature, whereas the y-axis’ of these graphs represent the 100 grams of water. Every line on the graph represents a different solute. If something is saturated (perfect ratio of solute to solvent amount-wise), it will be a point on the line of a solute. If something is supersaturated (too much solute), it will show up as a point above the line of a solute. If something is unsaturated (too little solute), it will show up as a point below the line of a solute.

An example of a question you could have is one where you are told that a certain amount of grams of a certain solute dissolves in 100 mL of water. However, you need to do a lab with only a value less than 100 mL of water. So, you would need to multiply the amount of mL of water you want to use by the grams of the solute that dissolves in 100 mL of water over 100 mL of water to get a value in grams. This value would be the amount of grams you need for the lower amount of water (so this value of grams should be less than the amount of grams that dissolve in 100 mL of water).

For example, the problem could say: At 10 degrees Celsius, 80 grams of Sodium Nitrate dissolves in 100 mL of water, but I want to do this lab with 60 mL of water.

Your calculation would look like: 60 grams H2O x 80 grams NaNO3/100 grams H2O. You would get 48, which is 48 grams of NaNO3 that would dissolve in/that you need for 60 mL of water.

If the lines of the solutes in the solubility curves graphs slope upwards, they are solids (because lines sloping upwards are moving across the x-axis, therefore increasing in temperature. Solids need heat to break their bonds/break apart and dissolve). Vice versa, gases would slope downwards. The steeper the incline of a line, the more soluble that solute is (because more of the solute is dissolving in 100 grams of water). The flatter the slope of a line, the less soluble the solute/substance is.

What are solubility rules?

Remember disassociation? Well, if you have ionic solids, when they dissolve in water, they break apart into their ions (different parts of their compounds). If a ion is positive (a cation), it is surrounded by the oxygens in water (as opposites attract), and vice versa. However, covalent solids do not have ions or a charge, so nothing happens to them! Water simply surrounds them, causing them to dissolve.

Solubility tells us how much of something/a substance dissolves, and solubility rules essentially tell us something is aqueous or solid.

There is a chart/table for solubility rules specifically concerning double replacement reactions. The top of the chart/table talks about/lists mainly water soluble substances (aqueous substances). Any exceptions in this part would make the compound solid. The bottom part lists mainly water insoluble substances (solid substances). Any exceptions in this part would make the compound aqueous.

All double replacement reactions must have a reason to go to completion (go from reactants to products and STAY as products (your reactants react with each other to form a product)), and not all double replacement reactions go to completion. To go to completion, a double replacement reaction must produce (have in their products) one of the following three things: either form a precipitate (solid), a gas, or water. If the double replacement reaction does not make one of these three things, it forms a reversible reaction (which essentially means that the product and reactants keep on switching back and forth (or the products do not stay products, they turn back into separate reactants) in an ongoing cycle). If a reaction is reversible, you must draw a double-sided arrow to signify so. The easiest way to know if something is reversible is if everything (all of the compounds in the equation/reaction) is aqueous (since that means that the products of the reaction are aqueous, which cannot happen, as double replacement reactions must form either a solid, gas, or water (liquid)).

So, essentially problems will look like having to write a balanced equation for a double replacement, having to determine each compound’s state of matter (whether it be aqueous or solid based off of the solubility rules chart), and therefore having to determine if the reaction is reversible or not (based off of its states of matter; if all products and reactants are aqueous, we know the reaction is reversible and needs a double-sided arrow).

What does like dissolves like mean? What are Lewis Structures? What is polarity?

What is molarity?

The symbol/unit for molarity is an uppercase M. Molarity defines the concentration of a solution (a solute to a solvent). The higher the number (of the concentration), the less water or dilutant there is in the solution (so if you have an extremely high number for concentration, your substance is not watered-down or diluted). The higher the concentration, the more solute there is (since solvents can be water, so higher numbers of solvents mean a solution is more diluted/watered-down). A lower concentration number means there is more solvent/the solution is more diluted.

The formula/equation for molarity is: M = moles of the solute/Liters (volume) of the solution. However, you most likely will be given the mass of the solute (so a value in grams), and you will have to convert from grams to moles using a moles conversion. You will also most likely be given the volume of your solution in mL instead of L, so you will have to divide your amount of mL by 1000 to get a value in Liters.

What is dilution?

Dilution is when a stock solution from a stock room that has been pre-made needs to be diluted (or watered-down so it is less concentrated). The equation for this is: M1V1 = M2V2 where M is molarity and V is volume (the unit of the volume does not matter- it just must be the same on both sides). The ones stand for initial molarity or volume (or the original stock solution directly from the stock room), whereas the twos stand for final (or the diluted solution).

For example, a problem that you could have is: What volume of a 2M CaCl2 stock solution would you need to make 500 mL of a 0.3M CaCl2? When you plug in the values into the equation, this is what it should look like: (2)(x) = (0.3)(500). ******Ask Zush for help!!

What is molality?

Well, to review, we know that MOLARITY can be affected by temperature, as we were told by Charles’ Law. Charles’ Law told us that temperature and volume have a direct relationship, so the hotter something is (increase in temperature), the more volume it has. If the moles of a solute (numerator of the equation to find molarity) is divided by a big number (small number/big number), it results in a lower molarity (smaller number).

Molality’s symbol, on the other hand, is a lowercase m in italics. The equation for molality is m = moles of solute/kilograms of solvent. Molality was created to solve temperature issues with molarity, as in this case, the mass won’t be affected (as mass isn’t affected by temperature). You will most likely be given two values in grams- the bigger number will always represent the solvent- and will have to convert to moles for the numerator (using a moles conversion) and to kilograms for the denominator (divide grams by 1000).

What are colligative properties?

The scientific definition for this is that it is a physical property that is affected by the number of particles but not the actual identity of the solute. So, essentially, what this means is that the more solute is added into a solvent, the more the properties of the solvent change. However, this solute could be anything from sand to salt- it is always going to change the properties of the solvent, and it will change more properties if more of it is added into the solvent.

There are four properties. The first one is the lowering of vapor pressure (essentially, the more solute you add, the more the vapor pressure lowers). The second property is boiling point elevation (the more solute you add, the more the boiling point increases/a higher temperature is necessary for substance to boil). The third property is freezing point depression (the more solute you add, the lower the freezing point/a colder temperature is needed to freeze the now bigger substance). The fourth property is osmotic pressure.

The formula for boiling point elevation is: Triangle Tb = Kbim in which the triangle Tb represents the change in the boiling point, the K is the boiling constant (which is dependent on the solvent), i is the number of ions (which comes from the solute- for example, covalent solutes have an i value of 1 since they have no ions. Ionic solutes’ value of i is the number of cations plus the number of anions (add its subscripts). m is the molality.

The formula for freezing point depression is triangle Tf = Kfim. More solute gets in the way of bonding, so the temperature needs to be colder, so bonds can stretch around (because heat breaks bonds and cold is the opposite).

What is equilibrium?

The constant for equilibrium is Keq (where K is the constant, and eq stands for equilibrium, or the tendency of something to bounce back and forth until something stops it). Reversible reactions (which we know have double sided arrows) have/are equilibrium. The formula for Keq is [products] divided by [reactants]. The brackets represent molarity, which means you must find the molarity of both products and reactants before you divide them (molarity represents concentration and is moles solute/L of solution).

These happen for double replacements

What is LeChatelier’s principle?