I did an experiment for extraction of acetylsalicylic acid from an aspirin tablet(632mg mass). The solvent used was dichloromethane. I got about 342mg from the 500mg that could be extracted. the melting point found for this sample was 118-120°C as compared to a pure sample's 134-136 °C.

since this m.p. is low, does this show that my sample is impure with dichoromethane? or with something else?

I think it shows contamination with something. What the contamination is remains to be determined. If you want a purer sample, I would recrystallize the product.

To determine whether your sample is impure with dichloromethane or with something else based on the low melting point, you need to consider a few factors.

First, let's discuss why the melting point of a substance is usually used as an indicator of purity. Pure compounds have characteristic melting points, which are defined temperature ranges where they transition from solid to liquid phase. Impurities in a compound can disrupt this transition, causing the melting point to be depressed or elevated.

In your case, the typical melting point range for acetylsalicylic acid is given as 134-136 °C. However, your extracted sample showed a melting point of 118-120 °C, which is significantly lower. This suggests that the sample contains impurities that are lowering the melting point.

Now, let's consider the potential sources of impurities. One possibility is that your sample contains residual dichloromethane. Dichloromethane has a boiling point of around 40 °C, so it would evaporate before reaching the high temperatures required for melting acetylsalicylic acid. If dichloromethane remained in your sample, it could give a lower melting point.

However, it is challenging to determine the exact nature of impurities solely based on the melting point data. To gain a better understanding of the impurities, you should employ complementary techniques for analysis, such as spectroscopy (such as infrared spectroscopy or nuclear magnetic resonance) or chromatography (such as thin-layer chromatography or high-performance liquid chromatography).

These techniques can help identify impurities and confirm whether they are from dichloromethane or other sources. Additionally, chemical tests like mass spectrometry or elemental analysis can also provide valuable information about the composition of the impurities.

By combining the data from various analytical techniques, you will be able to determine the nature and source of impurities in your sample more accurately.