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Tutor profile: Brenna C.

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Brenna C.
Chemistry PhD candidate at UT Austin
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Questions

Subject: Inorganic Chemistry

TutorMe
Question:

Considering the Jahn-Teller effect, which of the below best describes the geometry of [Cr(H2O)6]2+ A. Perfectly octahedral with no distortion B. Axial compression C. Axial elongation D. Cannot say

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Brenna C.
Answer:

You can pretty much immediately rule out option A: while this complex has 6 of the same ligand, which makes it a candidate for perfect Oh geometry, but we have to consider the influence of effects other than ligand field geometry to get the right answer. You should start by writing out the d orbital splitting for an octahedral complex and then fill in your electrons to see if you have an electronic distortion. We know (hopefully!) that Cr2+ is a d4 ion, so with filling in the electrons it should be pretty evident that you no longer have totally degenerate sets of orbitals regardless of spin state. Now that we've established that we do have JT distortion (and ruled out option A in the process,) we have to figure out what that distortion will look like. In this case, spin state does matter, but with a 3d metal ion and 6 weak field ligands we can pretty much assume high spin giving us one unpaired electron in the eg manifold, which will be put preferentially into the dz2 orbital resulting in the elongation of the L-M-L bonds along the z axis giving us option C. Axial elongation.

Subject: Chemistry

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Question:

For the reaction ?CO + ?O2 ---> ?CO2 what is the maximum amount of CO2 which can be formed from 6 mol of CO and 7.88 moles of CO2?

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Brenna C.
Answer:

6 moles When you're given 2 (or more!) different amounts of reactants, you can usually bet it's a limiting reagent problem. With a limiting reagent problem you need 2 things to start: 1. the balanced chemical reactions and 2. the amount of 2 or more reagents in moles. We have number 2 (if we were given masses in grams we would have to convert to moles before proceeding,) but we still need number one, or else we won't know how many moles of each reagent are used up in the reaction. Luckily it's a pretty easy one. Our balance reaction is: 2CO + O2 ---> 2CO2 Or, for every 2 moles of CO2 produced, we must react 2 moles of CO with one mole of O2. So we know that we must have twice as much CO as we do O2, but we have only 6 mol of CO compared to 7.88 mol of O2, so we know that CO will run out first and is our limiting reagent. Since the amount of CO we have limits the reactions, we must use the 6 mol of CO to determine the maximum amount of CO2 formed! We know from our balanced reactions that for every 2 moles of CO we get 2 moles of CO2, so with our 6 moles of CO we will get 6 moles of CO2

Subject: Basic Chemistry

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Question:

If 25.1 g of a pure compound contains 8.01x10^22 molecules, what is the molecular weight of the compound?

Inactive
Brenna C.
Answer:

188.7 g/mol When you don't know where to start, look at the units. We know that molecular weight is in the units of g/mol, so what ever we answer we get should be equivalent to some sort of mass in grams divided some number of moles, or ? g/mol= x g/ y moles What numbers were given in the question? 25.1 g and 8.01x10^22 molecules. It makes sense that the value we're looking for, molecular weight in g/mol, will be equal to 25.1 g/some number of moles, but where do we get the number of moles from? We weren't given a value in terms of moles in the question, but we were given a (very large) number of molecules, right? So all we have to do is convert molecules to moles by dividing by Avagadro's number, which is this case gives 0.133 moles. Now that we have a number of moles of a substance and the mass of that number of moles, we simply divide: 25.1 g/0.133 moles= 188.7 g/mol

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