Question

the crucible is not fired, as the procedure suggests, but had retained some impurities from previous use(or it could be oily smudges from fingers). the mass of the "dirty" crucible is recorded. however the impurities are burned off in the experiment. how does this affect the reported mass of the final product? explain

Answer 1

What is your SCHOOL SUBJECT?

Answer 2

This must be a chemistry question and you gave VERY little evidence of what you are working on. However, in general, let's make up some numbers.
15.00 g = clean xble
17.00 g = xble + our ppt
----------
2.00 g = mass ppt.

BUT suppose the xble is dirty, as the problem suggests, so it actually weighs 16.00 g = mass clean xble + dirt
When we add the ppt to the dirty xble, we have
15.00 clean xble
1.00 g dirt
2.00 g ppt
-----
Total 18.00 g

Now we fire it, the dirt burns off and it will weigh
15.00 g for clean xble
+2.00 for ppt
Total is 17.00
and when we subtract 17.00-16.00 = 1.00 g ppt which of course is wrong. We actually had 2.00 g ppt; therefore, the mass of the reported material is too low.