how to find empherical formula from percentages ?
Answers
Explanation:
Calculate the empirical formula.
In any empirical formula problem you must first find the mass % of the elements in the compound. ...
Then change the % to grams. ...
Next, divide all the masses by their respective molar masses. ...
Pick the smallest answer of moles and divide all figures by that.
Answer:
Notice below how I do the first problem with some attention to using proper atomic weights, as well as keeping close to the proper number of significant figures. Then, notice how I get away from that (as well as being real consistent with units) in the following problems.
Notice also how it really doesn't make much of a difference. The trick is to know when to do that and it comes only via experience. Generally speaking, in empirical formula problems, C = 12, H = 1, O = 16 and S = 32 are sufficient.
There are times when using 12.011 or 1.008 will be necessary. If you hit a problem that just doesn't seem to be working out, go back and re-calculate with more precise atomic weights. These problems, however, are fairly uncommon.
For what it is worth, one piece of advice on rounding: don't round off on the moles if you see something like 2.33 or 4.665. That first one can be rendered as two and one-third (or seven thirds) and the second one as four and two-thirds (or fourteen thirds). In a situation like that, you would multiply by three to reach the smallest whole-number ratio rather than dividing by the smallest.
I know it's easy to say, harder to demonstrate. Some of the problems below involve this thirds issue. Look for a problem involving citric acid. Just be aware that rounding off too early and/or too much is a common problem in this type of problem.
Example #1: A compound is found to contain 50.05% sulfur and 49.95% oxygen by weight. What is the empirical formula for this compound? The molecular weight for this compound is 64.07 g/mol. What is its molecular formula?
Solution:
1) Assume 100 g of the compound is present. This changes the percents to grams:
S ---> 50.05 g
O ---> 49.95 g
2) Convert the masses to moles:
S ---> 50.05 g / 32.066 g/mol = 1.5608 mol
O ---> 49.95 g / 16.00 g/mol = 3.1212 mol
3) Divide by the lowest, seeking the smallest whole-number ratio:
S ---> 1.5608 / 1.5608 = 1
O ---> 3.1212 / 1.5608 = 2
4) Write the empirical formula:
SO2
5) Compute the "empirical formula weight:"
32 + 16 + 16 = 64
6) Divide the molecule weight by the "EFW:"
64.07 / 64 = 1
7) Use the scaling factor computed just above to determine the molecular formula:
SO2 times 1 gives SO2 for the molecular