Learn about Junior Chemistry, Solutions 6, in this comprehensive video by bannanaiscool.
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Male Speaker: Concentration in moles per liter is really the standard that we operate by but sometimes concentrations can be done in different ways especially if the concentrations are very very tiny and then we will use units something like parts per million, you have heard of those before, PPMs and sometimes it sounds like its very scary calculation, its not really that bad just to make you aware of the formula, here is what it is. It's really a million part difference in mass between the solute and the whole solution total. So, a million part difference is well, if that unit stands for micro which is ten to the negative six, ten to the negative six grams of solute divided by grams of solution, that difference there is ten to the six difference and that's a million and so it's a part per million. Another way of writing that formula and easier one to understand and remember that is milligrams of solute divided by kilograms of solution. So, if you just know what your mass is of your solute into milligrams. If you know the mass of your solution, sometimes you have given the volume of the solution, so you actually have to have its density in grams per milliliter in order to find its mass. Then you can put in the kilograms of the solution here that divided by that is going to give you parts per million. Sometimes in chemistry, we have to actually come clean and when we are starting to develop a concept from its very rudimentary stage and then explain that work fully later we kind of actually have to first teach it, somethings are very simplistic and then later we tell you the truth, sorry but here is one of those things. When you take magnesium chloride and we make some solution of magnesium chloride, we call it MgCl2, and if its in solution we write aq, but you know there is no such thing as MgCl2 in solution because remember ionic compounds like to make ions in solution because they are electrolytes, you have seen that they conduct will they have to make charge particles, what actually happens is that the magnesium chloride doesn't even exist as a ionic compound anymore of course but it dissociate into its ions and so what we need to do is write dissociation equations to represent what's really happening. So if we have magnesium chloride in solution what are the two ions, Mg is two positive and Cl is negative one, they are both aqueous and all we have to do is go back and say of course that's two chloride ions we put it two there that's the dissociation equation for magnesium chloride. Now if somebody sent you, you know what I got here a 0.1 mole per liter solution of magnesium chloride. Well, you really actually have 0.1 mole per liter magnesium ions in solution and look at it, 0.2 mole per liter solution of chloride ions, double the amount in terms of moles in the equation, so we double the concentration in the dissociation equation to get the concentration of that ion. Ammonium sulphate dissociates into its ions when the two ions ammonium, NH4 positive plus SO4 with the two negative, those are the two ions just off the periodic table of your list of polyatomic ions that this turned into oh two ammoniums, two. That's how there is your balance equation for the dissociation of ammonium sulphate into its ions. Its just take the compound, break it into ions.