Solutions

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This is MCAT General Chemistry 2, we'll be covering solutions and solution chemistry. Solutions are made up of a solute dissolved in a solvent. So generally the solute is what dissolves and the solvent we have in greater amount. Also, the solute often undergoes a phase change. Whereas the solvent does not. Now there are several factors to solubility, or solubility is a measure of how much we can dissolve.

These are one, temperature. As the temperature goes up, solubility increases. However, if we had a gas in a liquid, as the temperature goes up, solubility would actually go down. So liquids in liquids, solids in liquids. As the temperature increases solubility goes up, the reverse is true for gases and liquids.

The next factor is polarity, meaning that like dissolves like. So if we have a non-polar liquid, it would be more soluble in another non-polar liquid. But a non-polar compound and a polar compound would become insoluble. And the last one is size, meaning that the bigger the molecule it is the less soluble it is.

So small molecules, since they can sneak in between the atoms of other compounds, are more soluble. Now if we add too much of a compound we get what's called precipitation, where we hit a saturation or solubility limit. So solids or the solute will start to drop out of solution. So we could say the solute salts out.

Now this would be for solids and liquids. What if you had a liquid liquid solution? When liquid liquid solutions reach their solubility limit, what they do is phase split. Essentially you'd get two liquid phases, now which liquid phase would be on top? That's right the one that's the least dense.

Now there's a couple units of concentration you're gonna want to remember. The first is percent mass. We get this by dividing the grams of the solute or grams of x by the total grams. And then if we wanted to convert it into an actual percent, we could multiply this by 100%.

As is, it would just give us a mass fraction. The next one is mole fraction, and this going to be moles of x divided by the total number of moles. Molarity, big M, indicates the moles of solute divided by the liters of solution. And molality, or little M, is the moles of solute divided by kilograms of solvent. Now, if we have essentially a very dilute solution that's in water.

These two become equal because the density of water is such that a kilogram of water and a liter of water are nearly identical a certain temperatures. So again, big M and little m will equal each other if were very dilute in water. And the last concentration unit is normality or N, witch equals to the equivalents of x per liters of solution.

Where an equivalent is essentially the moles of a particular component of a compound. So for instance an HCl, H2SO4 and H3PO4. We have one equivalent of hydrogen, two equivalents of hydrogen, and three equivalents of hydrogen.

Now there's a few polyatomic ions that you're just gonna have to memorize for the test. And there's no way around it. These are the ammonium, which is one of the few positive polyatomic ions. Acetate, carbonate, and bicarbonate. And notice when you see this bi on here, usually indicates that we've added an H.

So add H+ and we go from a minus two to a minus one. Next up, we have hypochlorite, the chlorite, the chlorate, and the perchlorate. A couple of things to notice here, ite is less than ate. So, ite will have two oxygens in the case of the chlorite. The chlorate has three oxygens.

Now hypo is the least amount so there is only one, and the Perchlorate has the most. So Hypochlorite has the least amount of oxygen's, Perchlorate has the most. Next up Chromate and Dichromate. Cyanide, Hydroxide, Nitrite and Nitrate, Permanganate, Peroxide, Phosphate, Hydrogen Phosphate and Dihydrogen Phosphate.

Silicate, Sulfite, Sulfate, Bisulfite, and Bisulfate. And the last thing we want to do is talk about oxidation numbers. It's easiest to calculate oxidation numbers if we remember a few general rules. Hydrogen normally equals a positive one, unless it's with a metal, Like a sodium hydride, it would be a minus one.

Oxygen is a minus two. So let's come up here to Hypochlorite, they said that the oxidation number of the Oxygen is minus two. All we need to then use the oxidation of the Chlorine to balance the net charge are minus here. So, if the Chlorine is plus one, plus one minus two would equal negative one of the charge.

So the Chlorine has a plus one charge. Now the chloride we have sum number plus two of the Oxygen's equals negative one. Or negative one plus four equals three. Because working backwards, three minus four is negative one. The Chlorate similar we'll have three Oxygen's negative six plus some number equals negative one.

This is five. And the last one, four times negative two equals negative one. So something plus negative eight equals negative one, this is seven. So as we added Oxygens to the Chlorine here we increase its oxidation from one to seven. How about the Chromate and the Dichromate?

Something plus four times negative two equals negative two. So this is negative eight, something plus negative eight equals negative two that would be six. For Dichromate we have seven times negative two equal negative two or something minus 14 equals negative two. Notice we have two Chromium atoms, so we need to say two times something or two times six would equal 12 minus 14 is negative two.

So the oxidation state of the Chromium didn't change. The last one let's do the Peroxide over here. We've got two Oxygens. So we would say we have two negative two charges or a net charge of minus four. However that's not the case, we've got a minus two. So in this case the oxidation number of Oxygen is actually negative one.

So in Peroxides the oxidation number of Oxygen is minus one not minus two. In most other cases it'll be a minus two, and if you set the Hydrogen to one, everything else you can calculate out as long as you try and keep this net charge equal to the sum of the oxidation numbers. Let's name a few ionic compounds. What would you expect the molecular formulas of Hypobromite, Perbromate, Bromate and Bromite to look like.

Remember, we said that ites have the least amount of Oxygen. Ates have more, and in general, hypo is the least and per is the most. BrO-, BrO2-, BrO3-, and BrO4-. So we'll just draw them up assuming they're analogs to the Chlorine oxianions.

And we'll say that the one with the least amount of Oxygen here is going to be the Hypobromite. The one with the most Oxygen is going to be the Perbromate. And then right underneath the Perbromate we'll have the Bromate. And then the second least amount of Oxygen the Bromite. Now let's name the following.

And we have a Copper plus the Sulphate ion. Now this is where it's important to remember the charges on these polyatomics. The Sulphate ion has a negative two charge, which means that in this particular compound the Copper must have a positive two charge. A lot of transition metals have multiple positive ions, multiple cations. And generally, we write their name with this positive charge in roman numerals so this becomes Copper II Sulfate.

We didn't know the charge of the Sulfate but it states it's minus one. It would incorrectly said this is a Copper. One plus or a Copper one Sulfate. And that would have been wrong. Copper one Sulfate actually looks like this. Because this two would go up to the top and we would have Cu+SO4 2-.

Then when it actually broke apart there would be two Coppers that would be Copper one Sulphate, this is Copper two Sulphate. Now our group one and two metals are slightly different. This right here is a, Phosphate, this is Magnesium. And we take these guys and we can flip them up to become the charges. And we'd say this is Magnesium two plus and the Phosphate three minus, so Magnesium Phosphate.

We don't write these ones, Magnesium two Phosphate. That's incorrect, because this is not a transition metal. Magnesium only goes into the two plus state, so this is just Magnesium Phosphate. Now in general, transition metals are either at plus one, plus two, or plus three.

You don't have to memorize all of them, however, those that are in the fourth period, or essentially that first row that's transition metals. Iron, Cobalt, Nickel, Copper, Zinc, those are gonna be the big ones to remember. Okay let's continue over here we have Sodium and this guy right here is the bicarbonate ion. So this is Sodium bicarbonate.

And the last one here FeCl3, this breaks in to Fe3+ Cl- and there's three of these Chlorines. This is going to be Iron three Chloride.

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