The following question is, as it will probably be obvious, a homework question, however I have absolutely no idea how to progress with it beyond what I have already done. This is the question:

$\pu{10 g}$ of an unknown metal reacts with oxygen to produce $\pu{16.58 g}$ of the metal oxide. $\pu{10 g}$ of the same metal reacts with fluorine to produce $\pu{25.63 g}$ of the metal fluoride. The valency of the metal is identical in the metal oxide and fluoride produced. Identify the metal after finding the mass of one mole.

My attempt: \begin{align} m(\ce{O2}) &= \pu{ 6.58 g} & \implies&& n(\ce{O2}) &= \pu{0.206 mol}\\ m(\ce{F2}) &= \pu{15.63 g} & \implies&& n(\ce{F2}) &= \pu{0.411 mol} \end{align}

I also made the equations, where X was the unknown metal: \begin{align} \ce{X + y/2 O2 &-> XO_y}\\ \ce{X + z/2 F2 &-> XF_z}\\ \end{align}

Any help with where to go next would be massively appreciated.


1 Answer 1


Your answer is Magnesium. How you get there requires you to first realize that the number of moles of Fluorine is double the number of moles of Oxygen. Since Oxygen will accept 2 electrons and Fluorine will accept one electron, you can deduce based off oxidation states and the number of moles that this metal has a 2+ charge when it is a cation. From this, you can write your chemical equations out X+1/2O2-> XO and X+F2->XF2 . Moving on, you can do some basic stoichiometry to determine you have .412 moles of X present and use that fact that X's mass is 10g to determine the molar mass of 24.272 g/mols. Then look at your Periodic Table to find what corresponds to 24.272 g/mol and Magnesium does. Hope this helps


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