A student rushed in order to complete this experiment quickly. In attempting to
ID: 1003565 • Letter: A
Question
A student rushed in order to complete this experiment quickly. In attempting to save time, the student made some procedural errors. Briefly describe the effect of each of the following errors on the calculated ratio of Mg to O (indicate whether the calculated ratio would be unaffected, too high, or too low). Thoroughly explain your reasoning.
a.) The student did not heat the crucible and crucible cover to constant mass before adding Mg and carrying out the reaction.
b.) The student failed to clean the tarnished Mg ribbon with steel wool, before using the Mg.
c.) Rather than adding 6 M HCl solution dropwise as directed, the student poured 3 mL of 6 M HCl solution onto the Mg in the crucible. The solution foamed, and some material splattered out of the crucible.
d.) When transferring the crucible from the hood to the balance, the student sneezed, blowing some of the fluffly product out of the crucible and onto the floor. Because it was impossible to collect the lost solid, the student continued the experiment using the remaining product.
e.) The student decided not to heat the crucible and contents to constant mass, following the reaction of Mg with HCl solution and atmospheric O2.
Explanation / Answer
a.) The student did not heat the crucible and crucible cover to constant mass before adding Mg and carrying out the reaction: This would lead to an error in the mass of Mg taken in the crucible as the actual mass would be lower than the mass we take due to the improper heating of the crucible and it's cover.
b.) The student failed to clean the tarnished Mg ribbon with steel wool, before using the Mg: Mg ribbon might have a coating of MgO on it's surface if left unused for several days. So, it might likely not result in the desired reaction if the ribbon is not cleaned with steel wool before using Mg.
c.) Rather than adding 6 M HCl solution dropwise as directed, the student poured 3 mL of 6 M HCl solution onto the Mg in the crucible. The solution foamed, and some material splattered out of the crucible: When concentrated HCl solution is added directly, it may cause spiling of the reaction mixture and it also doesnto ensure that the reaction is 100 % complete. HCl must be added dropwise in order to make sure that there is no loss in the reaction mixture.
d.) When transferring the crucible from the hood to the balance, the student sneezed, blowing some of the fluffly product out of the crucible and onto the floor. Because it was impossible to collect the lost solid, the student continued the experiment using the remaining product: There was a loss in the yield of the product as some amount of the product was lost during transfer from hood to the balance.
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