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Video transcript

a total of 29.95 milliliters of 1.25 molar hydrochloric acid is required to reach the equivalence point calculate the concentration of potassium sorbate when we put the brackets they're talking about concentration in the stock solution so let's just remind ourselves what the equivalence point is the equivalence point is the point at which at which all of the potassium sorbate has reacted with the acid with the tight rent so we have just gotten to the right-hand side of this net ionic equation so another way to think about it is it's the point in which because we are reacting for every for every mole of the potassium sorbate that we want to turn into sorbic acid we're adding a mole of the hydrochloric acid so one way to think about the equivalence point is if I've added one point if I've added a certain number of moles of hydrochloric acid that must be the exact number of moles of the potassium sorbate that we started out with we've we've added the exact number of molecules of hydrochloric acid as they were originally were a potassium sorbate so they can completely cancel out with each other so if we can figure out the number of moles if we can figure out the number of moles of hydrochloric acid here then we could say well at the equivalence point you or if you needed that number of moles of hydrochloric acid to reach the equivalent point then that means your original stock solution had that many moles of potassium sorbate and then we can use that to calculate the actual concentration so let's do the first part let's let's think about how many moles of hydrochloric acid this is right over here so I'll just write h CL and so if we have 29 29.95 milliliters and let's let's convert it into liters because if we have if our concentration is given in in in terms of molarity this is moles per liter so let's make sure we get our units right so let's multiply this times so we want to convert this to liters so we want leaders in the numerator and milliliters in the denominator so these cancel out so one liter is equal to 1,000 milliliters and that makes sense if you want to go from milliliters to liters you would multiply by 1,000 or divide by a thousand so this will give us this will give us the total amount of hydrochloric acid solution we've added in liters and now let's figure out how many moles that will be well we multiply that times the molarity times 1.25 instead of writing molar capital M I'm gonna write 1.25 moles per liter and what is this going to give us well let me get my calculator out and so I'm gonna have 29.95 divided by 1,000 that part right there times 1.25 times 1.25 is equal to and let's see I have three significant figures here four over here so the product I'm gonna have three so 0.037 four so zero point zero three seven four and the unit's all work out milliliters cancels out with milliliters liters cancel out with leaders and I have zero point zero three seven for moles I remember this is moles of hydrochloric acid so when they say that a total of 29.95 milliliters of 1.25 molar solution of hydrochloric acid is added they really added this many moles of hydrochloric acid so that means our original solution had that many moles of had that many moles of potassium sorbate so it had that many moles of potassium sorbate we can now figure out the original concentration of potassium sorbate so the concentration of our potassium sorbate c6h 702 is going to be equal to the number of moles of potassium sorbate our stock solution starts off with so that's going to be the same number right over here zero point zero three seven four moles / / well what was our original volume of our original solution let's see a student tried traits 45 point zero zero so we have four significant figures here student type traits forty five milliliters of the stock solution so the stock solution it originally has a volume of forty five milliliters so 45 milliliters if we wanted to write that in terms of liters remember the concentration the molarity here we wanted it in terms of well we want in terms of moles per liter so forty five milliliters is the same thing as zero point zero four five I can write zero the zero there so that have four significant figures here liters of solution and what is this going to be equal to and I just divided this by a thousand forty five milliliters is the same thing as forty five thousandth of a liter and so what do we have so we could take that number we just had and then we divided it by point zero four five and I could add two more zeros there but from the calculators point of view it doesn't make a difference and that's going to be equal to all right and how many significant figures do we have we have three significant figures so zero point eight three two zero point eight three two zero point eight three two moles per liter or we could say zero point eight three two molar did I write that right yeah eight three two and there and there you have it that's the initial concentration of the potassium sorbate in our stock solution
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