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## Chemistry library

### Course: Chemistry library>Unit 5

Lesson 4: Molecular composition

# Worked example: Determining an empirical formula from percent composition data

We can use percent composition data to determine a compound's empirical formula, which is the simplest whole-number ratio of elements in the compound. Created by Sal Khan.

## Want to join the conversation?

• how is there more chlorine than mercury if there is .73% and 200 moles of Hg and only .27% and 36 moles of Cl? •   OK, first some corrections. That was 73% by mass (not .73%) Hg and 27% by mass (not .27%) Cl. But more importantly, you have mistaken the number of moles (a measure of the number of atoms) of Hg & Cl for their atomic weights (a measure of the average weight of a collection of atoms of that element). But if you are still confused, and you like to reason by analogy, think of it this way....
Imagine you have a bag. In the bag, you are told, are nothing but feathers from a dove and fishing sinkers. Each feather weighs 1.0g; each sinker weighs 10.0g. The total mass of all the contents is 150g, and you are told that 2/3 of the mass is made up of sinkers, and only 1/3 of the mass is feathers. So there is twice as much mass of sinkers are there are of feathers. But how many (quantity) of each are there? 2/3 of 150g is 100g, so there are 100g of sinkers, or ten (10) fishing sinkers total. 1/3 of 150g is 50g, so there are 50g of feathers, or fifty (50) feathers total. So, while there is twice as much mass of sinkers, there are five times as many (quantity) of feathers.
• Why is Cl₂ called Chloride? And why does Sal say Hg "2" Chloride? What does the 2 mean? •  And the 2 denotes the charge of the cation, because transition metals have multiple oxidation states (which is essentially the charge of the atom within the molecule) (i.e. Fe can be Fe+3 or Fe+5), so in this case the oxidation number/charge of the mercury needs to be specified. If it were Hg 1 Chloride [not sure if this exists], the compound would be HgCl, versus Hg 2 Chloride which must be HgCl2 to balance.
• Why do we assume that the percent compositions are in given in mass rather than in volume or numerically? Why can't the percents be saying that we have a mole ratio just over 3:1? • Because atoms tend to differ widely in terms of mass.
If all atoms weighed the same then we could indeed use weight percentages to determine empirical formulas (formulae?), but, as Sal showed us in this video, there are two Cl atoms for each Hg atom, instead of the one Cl atom to each three Hg atoms that the percentages seemed to indicate.
In other words: There are six times fewer Cl atoms than it seemed. This is because Cl atoms are about six times LIGHTER than Hg atoms.
• This may have been answered in another video, but if you got a ratio of let's say exactly 1:1.5, would you round up or round down in the empirical formula? • So there are 2 Cl for every Hg, but if there's 73% Hg and 27% Cl, doesn't that mean there's more Hg than Cl in the bag, because 73% is larger than 27%? • There are two kinds of percents here: the mass fraction and the mole fraction. So what the percentage is depends on what kind of percent you're talking about.
HgCl₂ has one atom of Hg per 2 atoms of Cl. Thus by moles it is ⅓ Hg and ⅔ Cl.

By mass, since Hg is heavier the percentages are:
Hg: 1 mol × 200.59 g/mol = 200.59 g
Cl: 2 mol × 35.45 g/mol = 70.90 g
HgCl₂: 1 mol × 271.49 g/mol = 271.49 g
Thus, by mass:
Hg: 200.59 g / 271.49 g = 0.7388
Cl: 70.90 g / 271.49 g = 0.2612
So, by mass HgCl₂ is 73.88% Hg and 26.12% Cl.
• When I paused the video, I didn't look at moles, but just used the fraction of the weight divided by the atomic mass to get the relative amount of each, which came out to close to the same answer (a 2.1 to 1 ratio of Cl to Hg). Is it just a coincidence that I got it right, or is this an acceptable way to do this kind of problem? • If I follow what you meant by that, then it is no coincidence at all. However, you need to use very clearly stated units. If you get unclear about units, even if the numerical portion of your math is correct, your chemistry teach will most likely mark the problem wrong.

Incidentally, you cannot round off as much as Sal does in his videos. You have to respect the number of significant digits.
• Why was Carbon decided as the basis of the atomic mass unit measurement? • Is there a rule of the order of a molecule? if we have a non metal and a metal, we write the metal first, but what if a molecule contains 5 C, 4 H, 2 N and 1 O? Is it arbitrary? Is it C5H4N2O or..? (It seems like C tends to be written first?) • Why hydrargyrum"s name is mercury in this video? Thanks • why do we use empirical formula ? why don't we get the exact ratio of elements?
(1 vote) • Because in ionic compounds there are no discrete molecules, just ions bound to each other in a repeating pattern, thus there is no molecular formula possible.

For covalent compounds, we use the empirical formula as an intermediate step toward finding the molecular formula. So, until we have enough information to find the molecular formula, we have to make due with the empirical formula.

Also, particularly for extremely large organic molecules we may be more interested in the ratio of the different elements to each other rather than how many are in each molecule -- it just depends on what purpose we have in using the compound. After all, if we have say a protein with 25000 atoms in it, and all we are interested in is how much hydrogen we can extract from it by decomposing it, then we don't need to know exactly how many atoms are in each molecule, just the ratios will do quite fine.