In lab today, we measured the actual yield and melting point of aspirin that we had made in lab last week. Our actual yield was 2.87 grams. The theoretical yield was 2.9 g, so the percent yield was 98.96%. When we tested the melting point, the range was 106-118. Since the melting point of pure aspirin is 138-140, this means that our aspirin was impure. My question is what would have caused us to have a high percent yield, but low purity? What went wrong? The aspirin was completely dry, so it wasn't that there was water making the aspirin heavier, giving us a higher actual yield and therefore a higher percent yield, but obviously there was something in our aspirin that was making it heavier right? Or could we have measured the melting point wrong?
It looks like the Aspirin still had insoluble impurities in it, causing a melting point depression and a high percent yield. This may have occurred during the early stages of the crystallization process.
During the crystallization process, if the Aspirin and solvent are not filtered (Gravity filtration) before starting, then impurities such as dirt and other contaminants could bond to the crystallizing Aspirin. This error would translate all the way to your melting point and percent yield because the Aspirin is no longer pure.