[Aisha_18]

If an unknow has an observed melting point of 90 - 93 degress. Is the unknown compound A that has a melting poitn of 95.5 - 96 or B which has a range of 90.5-91. I was thinking it may be compound B beacuse the unknown has a broaden range and it may be due to impurites or incorrect determination of the melting point. But A seems far off to me.

What do you guys think?

[nj_bartel]

It's honestly hard to say.  Impurities lower melting point.  You have a moderately tight range and it's proximal to both MPs.  If you are to assume no operator error, you must pick compound A.  If operator error is a possibility, you would probably need to run a TLC, or NMR, etc.

[renge ishyo]

I agree with nj_bartel; it is a tough call. If you are pretty sure that your compound is at least a little bit impure then compound A would be your best bet because impurities lower the melting point; they don't raise it. On the other hand, if you are unsure of how accurately you determined the temperature when the melting started (i.e. it wasn't "really" melting at 93 degrees even though you started your measurement there) then it could be a very pure sample of compound B and that your measurement of the upper range of the melting point temperature was just a bit off.

While compound B may sound like the more attractive answer since it implies that you obtained a perfectly pure compound and simply read the thermometer wrong, it is also the least likely to be correct because it is rare for a beginning O.Chem student to obtain a compound that doesn't have at least some measureable amount of impurities in it. Therefore, my money is on compound A.