| Simplest Formula from Percent Composition | |
|
Problem
The mineral cassiterite is a compound of tin and oxygen. Chemical analysis of cassiterite shows that the mass percentages of tin and oxygen are 78.8 and 21.2, respectively. Determine the formula of this compound.
Solution
We want to find the number of moles of each element in order to determine the ratios of the elements and the formula. To make the calculation easy (i.e., let the percentages convert directly to grams), let's assume we have 100 g of cassiterite. In a 100 gram sample, there are 78.8 g Sn and 21.2 g O. Now, look up the atomic masses for the elements from the Periodic Table. The atomic masses are found to be:
Sn is 118.7
The atomic masses provide a moles per gram conversion factor. Using the conversion factor, we can calculate the moles of each element:
moles Sn = 78.8 g Sn x 1 mol Sn / 118.7 g Sn = 0.664 mol Sn
The numbers of moles of each element are in the same ratio as the number of atoms Sn and O in cassiterite. To find the simplest whole number ratio, divide each number by the smallest number of moles:
Sn: 0.664 / 0.664 = 1.00
The ratios indicate that there is one tin atom for every two oxygen atoms. Thus, the simplest formula of cassiterite is SnO2.
Answer
SnO2
Previous page > Sample Problem #1 > Page 1, 2
O is 16.00
moles O = 21.2 g O x 1 mol O / 16.00 g O = 1.33 mol O
O: 1.33 / 0.664 = 2.00
