A compound contains 69.94 percent iron and 30.06 percent oxygen. What is its molecular formula if the molar mass of the compound is 199.55 per mole?

1 Answer
anor277
Aug 23, 2016

#Fe_2O_3#

Explanation:

As with all these problems, we ASSUME, that there is a #100*g# mass of unknown compound.

We break this quantity down into atoms.

And, there is #(30.06*g)/(15.999*g*mol^-1)# #=# #1.88*mol# #O#.

And #(69.94*g)/(55.85*g*mol^-1)# #=# #1.25*mol# #Fe#.

If we divide thru by the lowest molar amount, we get an empirical formula of #FeO_(1.5)#, but because the empirical formula is the simplest whole number ratio defining constituent atoms in a species, we double this result to get WHOLE numbers, i.e. #Fe_2O_3," ferric oxide."#

Note that an examiner would be quite justified at A level to say that #"an oxide of iron has 70% iron content"# without quoting the oxygen percentage, and expect you to realize that the balance is oxygen.

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