Ex. Empirical Molecular
* C4H9 - C8H18
* NO2 - N2O4
* CH7O2 - C3H21O6
* They show only the simplest rations, not the actual atoms.
* Molecular formulas give the actual # of atoms…
* To determine an empirical formula we need to know the ratio of each element. This is why we use the table below; to gather all the information we may need to solve each problem.
Atoms | Mass | Molar | |||
Step # 1. Fill in the chart with what’s already given to you (the names and masses of the three atoms, plus their molar masses which you can find on the periodic table).
Step #2. Calculate the number of moles simply by following the conversion chart.
* 8.4g x 1mol / 12g = 0.7 mol
* 2.1g x 1mol / 1g = 2.1mol
* 5.6g x 1mol / 16g = 0.35mol
Step # 3. Find the smallest mole (0.35mol in this case) and divide it into itself and the rest of the mole #’s.
* 0.7 / 0.35 = 2
* 2.1 / 0.35 = 6
* 0.35 / 0.35 = 1
* If your answers are not “whole #’s you must multiply everything by a common #. For example:
* Say you end up with the numbers 1.0, 1.5, and 6.0… you would use the number 3 to change 1.5 into a whole # while still keeping the other two numbers whole as well. Thus you would end up with 2.0, 3.0, and 12.
= C2H6O
Atom | Ratio | ||||
Step # 3. You must be given the compound’s molar mass (in this case it’s 216.8 g/mol) in order to figure out its molecular formula. Besides the molar mass, you also know from your previous calculations that the empirical formula is PdH2. You then calculate the empirical formula’s molar mass (by looking at the periodic table). In this case it is 108.4. You can then divide 216.8 by 108.4 to figure out what number to use for the molecular formula’s subscripts. It is 2… thus you change Pd into Pd2 and H2 into H4. And there you have your molecular formula!
Informational and VERY helpful video:
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