Asked by Steve

Hello, I am having trouble with a question from my chem lab. Here is my data

Weight of empty crucible and cover = 21.113 g
Weight of crucible, cover, MgO and Mg3N2 = 21.906 g
Weight of crucible, cover, MgOand addition of H2O and heat =21.909 g

The change in mass is 0.00300 g (I got that much right)

Calculate the number of moles of Mg3N2. (i cant figure this out)
ive been given the equations:

change in mass (grams) = grams Mg0 - grams Mg3N2 reacted

and the equation
Mg3N2 (s) + 3H20 (l) --> 3MgO (s) + 2NH3
Answered by GK
MgO is 60.3% Mg. Mg3N2 is about 72% Mg
Mg3N2 with water added changes to Mg(OH)2 which is then is changed to MgO upon heating. For a small sample, the overall weight changes are quite small. You would need very precise lab results to make accurate calculations of the % of Mg3N2. There is some algebra involved. Your results indicate that you had only a trace of Mg3N2, the dark product mixed in with the major product, MgO, which is white. I don't see much point in doing elaborate calculations based on not so precise measurements about a trace of biproduct
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