Posted by peter on Monday, March 23, 2009 at 7:59pm.
Assume 100 grams. I will do one, what about 3 as an example
YOu have 72.4g Fe, 27.6gO
this represents in moles...
Fe=72.4/55.8= 1.30 moles
O= 27.6/15.99=1.73 moles
Now divide each by the lowest moles (we are looking for whole number ratios)
Fe=1.30/1.30=1
O=1.73/1.30=1.5
But we cant write a formula as
FeO_{1.5}
so we multiply the numbers by an integer until all the subscripts are whole numbers, in this case multiply by 2 to get
Fe_{2}O_{3}
All of these are done the same way.
Take 100 g sample. For Mn and O, that will give you 69.6 g Mn and 30.4 g oxygen.
Now convert to mols.
moles Mn = 69.4/atomic mass Mn.
moles oxygen = 30.4/atomic mass O.
Now you want to find the formula. Remember that Dalton suggested that molecules are made of atoms in the ratio of small whole numbers. Therefore, you want to convert the moles you have above into small whole number. The easy way to do that is to divide all of the moles by the smallest number. That will give you 1.000 for the smallest number and the other should be very close to a whole number . Round to a whole number unless it is something like 1 to 1.5 in which case multi;ly both by 2 to obtain small whloe numbers. Post your work if you get stuck.
can you help with the others as well?
Bob Pursley worked one for you and I worked another one. You can follow our examples and do the others. You need to learn to do these yourself.