# Finding the formula of the metal

• Chemistry
Homework Statement:
0.1 mol of metal X (Ar = 27) was burned in oxygen to give an oxide with mass of 5.1g. What is the formula of the metal oxide if it has a relative molecular mass of 30?

The choices are
XO
X2O3
XO2
X2O

But if I add up X2O the RMM is 70 not 30
Relevant Equations:
This question is confusing
I don't understand how the RMM could be 30
I haven't gone into a solution cause I found this question confusing

Borek
Mentor
Something is definitely wrong with the question and the answer given.

My bet is it should read: "0.1 mol of metal X (Ar = 27) was burned in oxygen to give an oxide with mass of 5.1g. What is the formula of the metal oxide?"

I found the mass of the X by 27x0.1=2.7g
I subtracted 5.1g-2.7g=2.4g of oxygen
So changed the mass of oxygen to moles which = 0.15
Ratio it with 0.1 mol of X and I get 1:1.5
So X:O is 2:3 ratio
There for X2O3
Which makes sense cause At=27 is Aluminium
And Aluminium Oxide is Al2O3

chemisttree
Homework Helper
Gold Member
Your logic is good but it doesn’t apply to what is asked. The truly confusing part is the “(Ar=27)” and the 5.1 g. :)

If we ignore that part, the rest of the question tracks normally.

If RMM=30:

XO case: atomic mass X = 14 (N, nonmetal)
X2O3 case: atomic mass X= -9 (not real)
XO2 case: atomic mass X=-2 (not real)
X20 case: atomic mass X=7 (Li)
This should be the end of the problem.

balanced equation

2Li (.1 mol) + 1/2O2 ——-> Li2O (.05 mol)

It does make 1.5 g of product. A dyslexic 5.1 g?

Last edited:
Borek
Mentor
Your logic is good but it doesn’t apply to what is asked.

Let's not confuse the OP further. Question asks what is the formula and that's exactly what OP calculated, so their logic applies. Answer depends on which parts of the contradicting information given in the question one considers erroneous and decides to ignore.