Learn about Junior Chemistry, Solutions 5, in this comprehensive video by bannanaiscool.
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Rob Lederer: One of the laboratory skills that you really must master at this level of chemistry is how to prepare a solution. So let's we say we have some copper II sulfate, we want to make a solution out of it. First of all, we have to know the concentration of our solution. We have to know what volume we want to make in order to work backwards and will do that in a calculation soon. To calculate the number of grams necessary for the copper II sulfate in the solution. So, we will weigh out a certain amount of copper II sulfate using a scupula of course to be able to transfer it perhaps into a beaker right away that has been taped on an electronic scale. So, once you got your solid in there, you want to dissolve into some water. So you pour some water in, stir it up with stirring rod. Let's say you want to make a 100 milliliter solution. Well, don't fill it up to the 100 milliliter mark on the beaker because, beakers are very inaccurate way of determining volume. Hey by the way, if you had some solid on here, on this stirring rod, you would then loose it, as you put it down on a lab bench. So you would never want to do that. Watch this. So, what you want to do is make sure that this solution gets prepared into a piece of glass where that will accurately give you 100 milliliters. So, what's that, well how about graduated cylinder. No believe it or not, not accurate enough. Well, how about one of the little graduate cylinders, we will just do it quite a few times. No, no that's even worse. Alright, what's you need to use is, this right here. That's called a volumetric flask. A 100 milliliter volumetric flask with only one line of line of measurement on it. That little line that you see there, carved around here. When the bottom of the meniscus of a solution touches the line here, to within two decimal places, you got yourself 100 milliliters. That's what you make a solution in. Now, what you wanted to do is then get a funnel and be able to transfer the solution that you are starting to prepare in here through the funnel into the flask, make sure that you get what you we call a wash bottle or as a beaker full of water here and rinse off the stirring rod and the beaker, put that into the volumetric flask. Rinse it off again. Because, if you wouldn't lick it, it's not clean. That's my rule. then you got to take a little wash bottle to rinse out that funnel and every time you rinse there, you did not pass that meniscus line because, if you did you got more than a 100 milliliter solution. So, may be the water level is up to here. what you want to do then, is bring it up to just below the line and then get some water in an eye dropper plink, plink, plink to finish the dirt job. When the bottom of the meniscus of the solution touches that line you got yourself 100 milliliters as precisely as you can determine it in a lab. You saw the pipette in the last demonstration. What's the pipette doing? It's helping to perform a dilution. A dilution where we are taking a known concentration initially and turning it into different concentration by adding more salt. So, here is a good question. 200 milliliter or 0.2 moles per liter. Now, by the way some times you'll see moles per liter written as big m beside the number. Don't get freaked out, doesn't mean anything other than concentration, moles per liter. Sometimes we do that at university. 200 milliliters or 0.2 mole per liter solution is diluted to 500 milliliters. So its concentration is going to go down. What's the new concentration? well, just think of it this way. You got to have to pour in more solvent to bring it up to 500 milliliters from 200 in order to get a new concentration. The concentration is going to go down. But you know the moles never changes. So the moles stays the same, before dilution and after dilution. Concentration times the volume equals moles. So the moles before equals the moles after or CiVi, concentration initial, volume initial equals concentration finally times