[dearborn333]

Hi everyone,

I am new to chemistry and this board and have an end of the chapter word problem that I'm stuck on.   The chapter only covered the general basics with nothing mentioned about conversions yet, so I don't know why this question is there.  But it may be on our chem exam, so I need to know how to work it out.     
Can someone help me please?   

 
The question is:   "Indium Oxide contains 4.784g of indium for every 1.000g of oxygen. In 1869, when Mendeleev first presented his version of the periodic table,
he proposed the formula In2O3 for Indium Oxide. Before that time, it was thought that the formula was InO.
What values for the atomic mass of indium are obtained using these two formulas?
Assume that oxygen has an atomic mass of 16.00. "
 
I am certain that my method of answering this question is wrong, but I need to know why it is wrong.   Can someone help me please?   


This is how I am thinking about it now:   

The atomic mass for indium is 114.8.
114.8 x 2 atoms of indium = 229.6

The atomic mass for oxygen is 16.00
16.00 x 3 atoms of oxygen = 48

229.6 / 48  = 4.78     

Indium contains 4.78 times the weight of oxygen. So I see where THAT number came from in the problem, but don't understand how to find the atomic mass of indum using the two formulas listed.  I want to say that the value of the atomic mass of indium using the new formula In2O3, can be found by taking the mass of one In atom (114.9 and multiplying it by 2 ( totalling 229.6). And to find the mass using the old formula, you multiply the atomic mass number by the number of atoms (1) leaving 114.8.   I am certain that this is incorrect though, because the answer has something to do with the 2/3 ratio, but I need help understanding why and how.

Any help would be greatly appreciated! 

Thanks!   

[Yggdrasil]

http://www.chemicalforums.com/index.php?topic=18854.0

[dearborn333]

Thanks Yggdrasil - that was a VERY fast reply!!!

The previous posting helps me understand why the ratio of In to oxygen is 2 to 3, but the question asks " what  values for the atomic mass of indium are obtained using these  two formulas?   Isn't the atomic mass just of the one atom?  I don't understand why the whole molecule is a factor.  What is the connection between the ratio of indium to oxygen , and the ratio of the old formula  InO, and the new formula In2O3? 

I see that the answer below is a 2/3 ratio,  but I'm blanking on this right now and need to see the connection before tomorrow.     
 
InO, atomic mass of In = 76.54
In2O3, atomic mass of In= 114.8

Thanks again!   

[dearborn333]

For now I will just accept that I need to take the atomic mass and multiply by 2/3 to get the

I need to ask one more question please.  The previous post listed the answer for the atomic mass of the old formula ( In )  as   76.54

When I multiple 114.8 x 2/3 I get 76.53.    Is this correct?   I know that this is  REALLY picky, but we submit our work via computer and get the entire question wrong if we are off by even one sig fig.  So I need to know whether I will be able to just multiply the atomic mass by the fraction. 

Thanks again!

[Yggdrasil]

Assume that you don't know the atomic mass of indium.  In order to find an atomic mass, you need two things, the mass of a sample of indium and the number of moles of indium in that sample.  You are given the mass of indium in indium oxide, sp you need to find the number of moles of indium in the indium oxide.

So, in a sample of indium oxide, say you have 1.0g of oxygen.  How many moles of oxygen are there?  Based on the formula InO, how many moles of indium are in that sample?  Based on the formula In₂O₃, how many moles of indium would be in the sample?