How to do this moles questionAn oxide of Y has a formula YO. When 2.16g of this oxide is heated strongly. it decomposes completely to give 120 cm^3...

When the YO decomposes, it will produce one mole of Y for
every mole of YO and 1/2 mole of O2 for every mole of
YO.

Since there are 22,400 cc of a gas in one mole, then in
this case there are 120/22,400 moles of oxygen produced.  This is equal to .0053 moles
of oxygen. That means you started with .0107 moles of
YO.

Now .0053 moles of oxygen = .0848 g which leaves 2.16 -
.0848 or 2.0752 g of Y.  Since this is equal to .01068 moles of Y, if you divide the
mass by the number of moles you get the molar mass of the element.  2.0752/.01068 =
194.3 g.  Looking at the periodic table, the element closest in mass to this is platinum
at 195.2 g/mole.

Therefore, the name of the oxide would be
platinum oxide.