Step 1 - Understanding the problem
We need to determine the mass of a sample which probably contains water as an impurity. Therefore, if we just weight the sample, we won't be able to separate its own weight from that of water.
What can be done, then, is heating the sample first in order to evaporate all the water, and only then weighting it. That's exactly what the procedure in the exercise has done.
The idea here thus is that we just need to subtract two weights:
[tex]m_{\text{sample}}=m_{\text{sample}+\text{water}}-m_{\text{water}}[/tex]Step 2 - Discovering the mass of the sample
We can discover the mass of the sample by simply subtracting the mass of beaker and cover from the final mass (beaker + cover + sample after heating).
This will give us the mass of the sample because the water has evaporated after heating:
[tex]m_{\text{sample}}=50.88-50.09=0.79g[/tex]The mass of the sample is thus 0.79g.
