Crucible and Oily fingers

[disneychannel]

Homework Statement

The crucible is handled with oily fingers after its mass measurement but before the ~3 gram sample of the hydrated salt is measured. Subsequently(after measuring the hydrated salt,the oil is burned off. How does this technique error affect the reported % water in the hydrated salt. Explain

The Attempt at a Solution

The measurement of the crucible will be accurate, but after touching the crucible with the oily fingers, it will contaminate the surface of the crucible. After weighing the hydrated surface and then burning the crucible, it will seem that more water was lost because the oily fingers would have increased the mass of the hydrated salt.

I'm kind of having second thoughts on my answer...part of my mind thinks it will no be affected? HELP!

 

[Moonbear]

Your initial response is correct, now convince yourself of it to make sure you understand. Take it one step at a time to determine where the error is. If it helps, make up a number for the mass of the oily fingerprint so you can track its effect quantitatively.