A student rushed in order to complete this experiment quickly. In attempting to
ID: 536289 • 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 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 O_2.Explanation / Answer
a) Not heating the crucible will have no effect on the Mg/O molar ratio of MgO, but it will effect the weighing results. The object of preheating the crucible is to drive off the moisture and wetness.
b) Magnesium will develop a dull oxide coating when exposed to atmosphere (MgO). We need to clean off the oxide coating to ensure we are weighing just the magnesium. Due to oxide layer, the Mg/O ratio will be lower.
c) Since the solution splattered out, the amount of Magnesium is reduced in the crucible. So the Mg/O ratio will be lower.
d) If some of the magnesium oxide has been lost from the crucible, the percentage composition calculation of magnesium would be too high. This is because, when the magnesium oxide is lost as light, we would notice the percentage of magnesium to be quite larger.
e) It won't affect the ratio of Mg/O because this is just to remove the excess moisture.
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