One almost ALWAYS weighs by difference (ALWAYS if you're in quant lab--perhaps not so often in other course where you can scoot by with a little less precision and accuracy)l Let me exaggerate with an example.
Let's suppose we want to weigh a sample for analysis. We place some powder on the pan and it weighs 2.00 g + 1.00 g = 3.00 grams. So we record a mass of 3.00 g in out lab book. (And your instructor chews on you for placing a sample directly on the pan but play along with me here). Now, what you don't know is that the weights are not calibrated properly. The 1.00 g weight actually has a mass of 1.00 grams; the 2.00 grams weight actually has a mass of 1.00 g. So the sample we just weighed has a real mass of 2.00 grams but we've recorded it as 3.00 grams. We are in big trouble already.
Suppose we weigh the sample by difference.
First we weigh a sample with container. We use the 1.00 g weight (that is accurate) and the 2.00 gram weight (which actually weighs 1.00 g) and a 5.00 gram weight that is accurate. The total mess weighs (we think) 1 + 2 + 5 = 8.00 g. Next we pour out some of the sample and weigh again. This time it weighs 2.00 (which actually weighs 1.00 but it is calibrated as 2.00) + 5.00 so we record it as 7.00 g. Weighing by difference gives us 8.00-7.00= 1.00 g (EVEN though we used the miscalibrated weight). The safe thing to do is weigh by difference. Errors in handling, calibration, etc, tend to cancel.
Why not distill to bone dry (or near bond dry) conditions. The organic materials will likely decompose at elevated temperature.
what could be possible answers to these two questions.....1 why determine the amount of solvent by difference and not by weighing the final distillate?????
why not distill off all the solvent at the very beginnig and then separate the solid residue of benzoic acid and benzoin recrystallisation
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