[Trish]

This is a homework question I'm absolutely stumped on. It's in 6 parts, and I got two correct. But I honestly can't figure out how to get the others. Here's the full question:

A 0.160 M solution of an enantiomerically pure chiral compound D has an observed rotation of 0.20° in a 1-dm sample container. The molar mass of the compound is 128.0 g/mol.

(a) What is the specific rotation of D? - I got this correct with 9.77.

(b) What is the observed rotation if this solution is mixed with an equal volume of a solution that is 0.160 M in L, the enantiomer of D?

(c) What is the observed rotation if the solution of D is diluted with an equal volume of solvent?

(d) What is the specific rotation of D after the dilution described in part (c)? - I got this correct with 9.77.

(e) What is the specific rotation of L, the enantiomer of D, after the dilution described in part (c)?

(f) What is the observed rotation of 100 mL of a solution that contains 0.01 mole of D and 0.005 mole of L? (Assume a 1-dm path length.)

I converted the 0.160 M into 0.02048 g/ml and I've plugged in what I believe to be the necessary numbers into the specific rotation equation to find the observed rotation, but it's still coming out wrong. I've had no luck solving the remaining questions. I don't want an answer spoon-fed to me, but if somebody could tell me what I need to do for the remaining questions (and possibly give me the equations I need), then that would help me immensely.

[mjc123]

What equations do you need? You have used an equation to get the specific rotation from the observed rotation; can't you use the same equation to get the OR from the SR? I think you are probably getting confused with your mixtures and dilutions.
If the SR of D is 9.77 [units?], what is the SR of L?
So what is the OR of a solution containing equal amounts of D and L?
If you got (d) right, why can't you get (e)?
What is the effect of concentration on OR?

[Trish]

I've used the specific rotation equation for this problem, and my answers are still wrong. Also, I've plugged in 9.77 for (e) and it was flagged as wrong. And I thought the OR were equal for the problems that had solutions containing equal amounts of D and L, but that was also flagged as wrong.

[Borek]

Try to answer mjc123 questions.

[Trish]

Try to answer mjc123 questions.

Except I did answer their questions. I put L's SR as 9.77 and it was flagged as wrong. I put +0.20 for the OR of L and it was flagged as wrong. I'm using the specific rotation formula when I try and calculate these problems and they're coming back as wrong. I don't know what I'm doing wrong. I've checked and read my organic chemistry textbook, but it only gave me this one equation.

Everything that mjc123 suggested I do I have tried already, and it's still coming out as wrong.

[Borek]

I put L's SR as 9.77 and it was flagged as wrong.

So you say it is identical to SR of the D form? What is the difference between D and L then?

[Trish]

I put L's SR as 9.77 and it was flagged as wrong.

So you say it is identical to SR of the D form? What is the difference between D and L then?

I'm starting to get confused. I feel like this board is giving me the runaround and is not helping me in any way. L is the enantiomer of D, so I would assume that they are identical. Or at least mjc123 told me they were identical. But when I put 9.77 for the answer for the SR for L it was flagged as wrong. Everything I have written in the OP came from the problem. I've been working on this problem for 4 days and I need help. People have questioned me about why I haven't tried certain steps, but I have tried them and I'm still getting the wrong answers.

[Borek]

Seems like you are missing the most basic property of these substances, we assumed you know the basics, hence the problem.

L is the enantiomer of D, so I would assume that they are identical.

If they were identical they would be not enantiomers.

They are mirror reflections, so they rotate polarized light by exactly the same amount - but in different directions. Say, for some solution of D OR is 22 degrees - then equivalent solution of L will have OR of -22 degrees.

Can you think of a way of expressing this difference between L and D in terms of their SR values?

[mjc123]

Or at least mjc123 told me they were identical.

I said nothing of the kind. You may have thought I was implying that they were identical; actually I was trying to prompt you to remember that they are the same magnitude with opposite sign. I say "remember", but did you ever know? As Borek says, we assume you have studied the basics of this topic, otherwise you wouldn't be given a homework question on it. Would you?
That's why it's so helpful for us to see your working; it helps us to see what you know or don't know, and where you have made a mistake. Just saying "I got it wrong, please help me" isn't very useful.

[Trish]

I'm done with this board. You guys aren't interested in helping me. All you've done is give me the runaround when I've told you repeatedly what I have done that hasn't worked with this problem. I don't know how many times I have to tell you guys that I've done the steps you've told me to do. I don't know how many times I've told you that I've plugged the provided numbers (i.e. 9.77, 0.20, and even 0.02048) into the specific rotation formula many number of ways and keep coming up with the incorrect answers.

No, just about the only thing you guys are interested in is lecturing me about why I don't know certain things. I didn't come on this board for the answers to this question, I just wanted help. But the whole lot of you wasted my time instead. Thanks for nothing.

[Dan]

You have to meet us half way and show your working. All you gave was vague descriptions of plugging what you thought were the right numbers into what you thought was the right equation - that is not enough to help you with the Socratic method of teaching that we are bound to use by the forum rules.

You can try again, showing all calculations in full, or you can take your question elsewhere. The choice is yours.

[Trish]

I've told you everything I've done. I've followed the rules of this forum by doing the work myself and explaining what I'm doing wrong. You guys have asked me why I haven't tried certain steps, and when I've told you I have done those steps, then you accuse me both of not knowing anything and for being "too vague" in how I ask for help. Socratic method stipulates that you think and stimulate my thinking in order to help me find the right answer, but you haven't done that. The only thing I have gotten out of this board is that you're not interested in helping me. You'll just keep giving me the runaround until I get fed up and go elsewhere with my question, which is exactly what I intend to do.

Once again, thank you for wasting my time and for giving me zero help.

[Borek]

Actually we are trying to make you THINK. We gave you everything you need, you just refuse to make an effort.

If you keep plugging the same numbers in the formula, you will never get two DIFFERENT answers - and as I told you, D and L rotate polarized light in DIFFERENT directions. Different directions mean different sign of the measured angle.

[Trish]

I guess busting my butt over this one problem for what is now 5 days doesn't constitute "making an effort". And how about you actually read my posts before you assume I'm not doing anything? I've stated more than once that I assumed that different direction meant different sign, so I plugged in the angle with a negative in front, because it was positive on D. But this was still flagged as wrong. Like I've also stated repeatedly, what people here have suggested to me to try critical thinking that I have already tried before even coming here. But it hasn't worked for me.

So I have put forth considerable effort into trying to figure out why I'm continuously getting this problem wrong. It's just that none of you want to put forth any effort to help me, so you ignore what I say and instead accuse me of not knowing anything and of not working hard enough.

[Corribus]

If nobody was interested in helping you, there wouldn't be well over a dozen posts in the thread. And after that, you still haven't clearly explained how you solved the various parts of the problem - even those you've said you got right. This is all very important for us to try to evaluate what you're doing wrong. For myself, when I try to help someone I like to reproduce what the person did, even those parts they say they did right.

You are frustrated. That is apparent. But people here are only trying to help.