Learn about Junior Chemistry, Stoichiometry 1, in this comprehensive video by bannanaiscool.
Read the full transcript »
Male Speaker: You may want to thrown up your hands oh my goodness, where do I start. Don't look, just be patient, go through this balancing in chemical equation first and then everything falls into place. Really, it's quite logical. Here is the reaction, silver nitrate, potassium chloride, double replacement reaction K goes with the NO3, Ag with the Cl and here is what you get, the all positive one and negative one charge so they are easy to put together here, cations before anions and look AgCl is a solid, it's a precipitate, the question said we want to find out how many grams of precipitate we can form if we have lots of this reacting with only 2.6 moles of this, so we call this the limiting reagent or reactant, the thing that we are limited by that tells you how much it can actually form -- Now look if we can somehow take this information here and turn to get the information here then we can find the mass here, lets use logic, this is a one to one what ratio, mole ratio. This says for every mole here we have one here, so for every 2.6 moles here how many can you form there. Yeah 2.6 and so hey if you don't look moles of silver chloride can you find the mass, the molar mass, all of this can be done in one calculation sentence when we use dimensional analysis and unit cancellation, so this is how we do it. 2.6 moles of KCl, alright, we don't want moles of KCl, we want moles of silver chloride, if this is a mole ratio, so it's perfect, we get greater the moles of KCl and we want to keep moles of AgCl, you understand while this is kind of a redundant step because it is just a one to one ratio so its one to one, so that will do it. It so got a nature and this cancels properly, so that's necessary, oh I want to do that. Now the moles of KCl cancel, we got moles of silver chloride but we don't want that. We want the mass of the silver chloride, we want mass of AgCl, we want the moles of AgCl to cancel out, what's the molar mass of AgCl, we look at up in the periodic table, one Ag and one Cl and you get 143.32, 143.32 grams every time you have a mole. Look at what we just did, this stoichiometry and that's what it is called stoichiometry, this cancels out here and here it cancels out moles of AgCl here and here to leave us with an answer of grams of AgCl, which is what we want and that mass is 372.632, 372.632 grams of AgCl but you know as well as I do you cant keep that, you got two significant digits here by the way these are infinite numbers of significant digits, exact numbers have an infinite number of significant digits, this has five, and exactly one mole so that's -- again, keeps a least number 5 to 2, how to keep two, 372.632, one two, 3.7263 two times ten to the positive because we are making this number small so we have to make the power of ten large. But look at the two significant digits, 3.7 times ten square grams of AgCl, there is the answer, follow along with that again, you can review it, look at it and make sure that you understand this mole to mole ratio in the equation how that translates here into doing stoichiometry.
Copyright © 2005 - 2015 Healthline Networks, Inc. All rights reserved for Healthline.