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Pure magnesium's formula would just be Mg because all elements except for 7 nonmetals are just left alone when they are by themselves in a formula. The 7 diatomic elements( means they have to have two of them without another element attached to it aka. a subscript two after it when it's by itself) are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. An easy way to remember the diatomic seven is that when looking at a periodic table if you trace over them from nitrogen over to fluorine and down to iodine all of those elements are diatomic + hydrogen.
And your unbalanced and balanced equations are correct.
(sorry I went on a tangent with the diatomic rules hopefully it will help you in the future though)
N = ?
T = 449 K
V = 58.35 L
P =2.97
R = 0.082
Use the clapeyron equation:
P x V = n x R x T
2.97 x 58.35 = n x 0.082 x 449
173.2995 = n x 36.818
n = 173.2995 / 36.818
n = 4.70 moles
hope this helps!
Always younger than the rock it intrudes