In this experiment you determined the mass of oxygen (that combined with a weigh
ID: 494477 • Letter: I
Question
In this experiment you determined the mass of oxygen (that combined with a weighted mass of magnesium) by subtraction: mass of product minus mass of original magnesium = mass of oxygen combined. As a result of this procedure, anything that was in the crucible at the end of the experiment, along with the magnesium oxide product, would cause an error in the figure that is recorded as "mass of oxygen." Would extra mass in the crucible cause the "mass of oxygen" to come out too high or low? Explain. The correct formula for magnesium oxide is MgO, a 1.0 to 1.0 ratio. But sometimes in this experiment the ratio of Mg to O comes out too low. (Example: 0.9 Mg to 1.0 O) In that case, it means that there was too much oxygen relative to the mass of magnesium. At other times it comes out that the ratio is too large. An example would be: 1.2 to 1.0 (Mg to O). In such a case it must be that there has been too little oxygen (or too little weight at the end of the experiment, which registers as too little oxygen.) In each case below, decide whether the situation described would lead to a calculated ratio of too much oxygen, or too little oxygen, and explain your reasoning. Putting in more water than is needed and then not drying out this excess water. Having some Mg(OH)_2 left in the crucible instead of converting it to MgO. Having some Mg_3N_2 in the crucible as product, instead of pure MgO. Forgetting to weight the cover along with the crucible and contents at the end. Letting a lot of the dense white smoke escape from the crucible during the burning. Here is some data from a similar experiment, to determine the empirical formula of an oxide of tin. Calculate the empirical formula according to this data. Mass of crucible, cover, and tin sample 21.76 g Mass of empty crucible with cover 19.66 g Mass of crucible, cover, and sample after heating 22.29 g
Explanation / Answer
1.
The extra mass in the crucible will cause the mass of oxygen to come out too high. Infact the weight of the extra mass in the crucible will be added to the weight of oxygen. This will cause the mass of oxygen to come out too high.
2.
a. Too much oxygen. The weight of water will be added to the weight of oxygen.
b. Too much oxygen. The weight of water (present in magnesium hydroixde as -OH groups) will be added to the weight of oxygen.
c. Too little oxygen.
Magnesium nitride molar mass = 101 g/mol
Magnesium oxide molar mass = 40 g/mol
Magnesium nitride formula unit contains 3 Mg atoms
Magnesium nitride molar mass (per Mg atom) = 101/3 = 33.7 g/mol
This will cause the mass of oxygen to come out too little.
d. Too little oxygen.
The weight of oxygen will be less by an amount equal to the weigh of the cover.
e. Too little oxygen.
Some of MgO may escape.
This will cause the mass of oxygen to come out too little.
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