Question

why is neglecting to do a final rinse of the stirring rod with poluceman into the beaker containing the accumulated copper of error in the experiment. What would the effect of this error be on your mass of copper? what would the effect of this error be on the ratio of mol Cu or Mol Fe
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i cannot read the hand writing

Answer 1

Ans. A stirring rod with policeman may retain some Cu-precipitate while scrubbing the inner walls of the beaker or while transferring the precipitate from reaction vessel to a suitable place like crucible.

The following errors may appear if the stirrer policeman is not rinsed appropriately-

I. Error in mass of Cu-precipitate: Because some Cu-precipitate is still retained (present) on policeman, the observed mass would be lesser than the actual mass.

For example, assume there is total of 1.000 g Cu-precipitate initially present in the beaker. If you use a policeman to transfer the precipitate to a crucible, 0.100 g precipitate is retained on policeman. If you rinse it only once, say 0.050 g precipitate goes back into beaker. However, there is still 0.050 g precipitate on policeman, which would be lost if the policeman is not rinsed several times in the beaker.

Therefore, if you don’t give a final rinse to the policeman back in the beaker, there is unaccounted loss of Cu-precipitate. Thus, observed mass (say, which would way after drying) would be lower than the actual mass of Cu-precipitate produced.

II. Error in moles of Cu: Moles of a substance is directly proportional to its mass, i.e. greater is the mass, greater would be number of moles of a chemical species.

Due to loss of Cu-precipitate, there would be lower mass of Cu obtained. So, the calculated number of moles of Cu would be lower than the actual value.

III. Error in mass moles of Fe: The exact output could be presented if the relation between Cu and Fe in the beaker (i.e. the reaction) would have been specified in the question.

For example If there is formation of Fe-precipitate in beaker, the effect would be similar to that of Cu. That is, loss of Fe-precipitate would lead to lower mass as well as lower number of moles of Fe that you can weigh and calculate.