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# 2015 AP Chemistry free response 2a (part 1 of 2)

Moles of ethene produced from dehydration reaction with ethanol. From 2015 AP Chemistry free response 2a, part 1 of 2.

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• ugh... did I miss something? The last video I watched in the chemistry play list was Limiting Reagent Stoichiometry, when did all of this "torr", "catalyst", "K*mol", and other stuff some from? Can someone post links to vids to help out maybe?
• Why can you use the total volume of the gas produced, 0.0854L, as the volume of Ethene gas? Isn't that volume including the water vapor as well? Why wouldn't you need to subtract part of that volume to find out the actual volume of ethene gas in the tube?
• The question states the final values (volume, pressure, temperature) of the collected gas at the END of the reaction, so we must consider the problem independent of time. This means the temperature can be considered constant - as this problem would become much more complex with time-dependance and thermodynamics. As to the question about volume: we are able to use the total volume since we are using the partial pressure of ethene in our Ideal Gas Law equation. Since the temperature, pressure, and volume are assumed 'constant' the number of moles of each gas is dependent only on the partial pressure of the respective gas. If we used the total pressure of the gas collected, then the solution (to the Ideal Gas Law eq.) would give us the moles of both the ethene AND water vapor at the END of the experiment.
• can't we simply find the number of moles of ethanol and then use stoichiometric ratio to find moles of ethene and then find mass of ethene produced?
• If we assume that the reaction goes to completion, then yes, we can just use stoichiometric ratio to calculate the the amount of our products. However, since question (a) part (i) asks us to find what is actually produced, it’s safe to assume that the reaction did not go to completion. In this case, we have to use the information given in the question to figure out how much ethene is produced, which is why Sal takes the steps that he did.
• We have considered the pressure of vapor water but how about the volume of vapor water? Shouldn't the volume of ethene smaller than 0.0854 because some volume of the collected gas belongs to vapor water?
• While it’s true that the water molecules in the collection tube would take up some volume, when thinking of gasses in the “ideal” sense, we usually assume that the individual gas molecules do not take up any space in the container, since the gas molecules only make up a tiny proportion of the overall volume of the container.
• What's vapor pressure or partial pressure and how did we know the total pressure is the sum of the partial pressures of ethene and water?
• What is a "catalyst" and an "atmosphere" and "torr"?
• A catalyst speeds up the rate of reaction

Atmosphere and torr are two different units of pressure
• So this question is not related to the information in the video

I checked the chemistry AP course and found it the same as the general course.
Can anyone explain to me what's happening? Is the general course AP?
• What is atm and torr. these are new units for me and I'm quite confused?And what is a catalyst is it somewhat like a precipitating agent?
• They are both units of pressure

A catalyst is something used to speed up chemical reactions
(1 vote)
• Were you not meant to times by 305 K in the last calculation rather than divide?