# How to know the formula of different compounds? [duplicate]

For example, if I didn't know the compound tin chloride, and I was asked to write out its formula, how would I know how to? Because it seems unreasonable to know the formula of every compound in existence. Many thanks for the help!

• You should have a look on possible and common oxidation states of tin and chlorine and then you should be able to figure out the formula of tin chloride. – pH13 - Yet another Philipp Jul 13 '15 at 10:24
• But if you didn't know what tin o r chlorine was how would you determine what to write if you had no books or data or anything to refer to? – Technetium Jul 13 '15 at 10:37
• @Joel That's a pointless question. If you wouldn't know what "addition" or "subtraction" mean you couldn't answer questions like "1+2=?" or "3-5=?" ... you simply have to learn certain things to answer questions. – pH13 - Yet another Philipp Jul 13 '15 at 10:47
• I do not agree that this question should be closed. The answer could reasonably be short, as @PH13 subtly mentions it. Lacking research != Too many possible answers/too long answers. – M.A.R. Jul 13 '15 at 16:20
• No one knows every formula, however there are rules and algorithms to find such formulas. – Asker123 Jul 14 '15 at 0:44

In my experience if someone says something like tin chloride they are usually insinuating you know the most common oxidation state that metal is found in when complexed with the chloride atoms. This is a bad practice of them if they haven't explained further but I digress... In this example with the metal center of the complex being tin, the most common oxidation state is +2. This means the formula in its neutral state would be $\ce{Sn^{+2}}$ with 2 chlorides with a (-1) charge making the molecule neutral at $\ce{Sn(II)Cl2}$.

Different effects cause common oxidation states for specific elements. These are outlined further with Mendeleev's Periodic Law. It breaks down into how shielded the nucleus is by electrons, and how big the radius of the atom is.

A further explanation of tin's common oxidation state calls upon knowledge of the inert pair effect, where elements to the right of the second and third row transition metals such as: In, Tl, Sn, Pb, Sb, Bi form compounds that are less reactive than one would think where the oxidation state is 2 less than the expected group. This is caused by the $\mathrm{ns^2}$ electrons being relatively inert. The cause of that nonreactive state is because of two reasons specifically:

1. Larger-than-normal effective nuclear charges in these elements.

2. Lower bond energies in their compounds.

A good book to use referencing this subject further is Descriptive Inorganic, Coordination, and Solid-State Chemistry by Glen E. Rodgers. This is the book I own and used for when I studied periodic trends, and inorganic chemistry as an undergraduate. It's a great book for this subject, among others.

Also there are commonplace names that are used as somewhat of a slang for certain compounds used in research frequently. These can be learned through experience, or you can just google common names... All formal names are dictated by the rules set forth by the IUPAC. Learning those rules would be good idea. Also keep in mind that in some research projects you will encounter molecules that have informal nicknames too.

Actually, the rules for naming compounds have been converging on the notion that you should be able to construct the compound from the name. How you do this is simply research and practice. There are sites for inorganic and organic naming conventions. Regarding your example, I immediately knew you were talking about an inorganic salt of tin, and I further knew than tin tends to be in the $+2$ state, resulting in the formula $\ce{SnCl2}$. But I also know lots of other possible formulas simply based upon the linked rules. You may not know everything, but you can know a lot simply by learning some basic rules.

Also, @Ryan Lee's answer points out that "tin chloride" is potentially ambiguous, and modern convention has been changing in a direction to make this more clear. The preferred name for this compound is tin(II) chloride, which emphasizes the the fact the the oxidation state of tin is $+2$. Most naming conventions have been formalized so that they encode a way to completely reconstruct the molecule. Nonetheless, tradition exists, so we often still call that solvent acetone rather than propan-2-one, the latter being much more structurally descriptive (i.e. ketone functional group substituted at the 2nd carbon of propane).

The other answers here have already given good responses to how you would find the answer to the specific example you gave. I want to focus on the question and this particular quote:

...it seems unreasonable to know the formula of every compound in existence.

You're absolutely right. In principle, we're supposed to have formal naming systems (thanks trb456 for the first two links) that describe exactly which molecular formula you're dealing with. Unfortunately, chemical naming conventions are a bit of a mess, with multiple names for every molecule. So what's a chemist to do?

The answer is that instead of learning the formula for every molecule in existence, you learn to extrapolate and make some pretty good guesses off of what you know. For instance, let's say someone's talking about lead chloride. Well, I don't know what the formula for that is! It could be any number of chlorines!

Luckily, I do recall that $\ce{PbO}$ is often used in microscopes. And since oxygen can only take on a 2- charge, that means that lead has to be +2, so the formula is $\ce{PbO2}$!

Oh wait, but I also remember something in my textbook about a $\ce{PbO2}$ compound. That means that there's two possible forms of lead chloride, $\ce{PbCl2}$ and $\ce{PbCl4}$. And now I've narrowed it down to two possible formula, which the Wikipedia disambiguation page helpfully shows us are the only two.

As to which one is specifically being talked about (and you'll have to believe me on this), I would be inclined to believe that it's the lead(II) variant simply because you see lead(II) so much more often than you see lead(IV).

Another trick you can use is to look at common naming conventions and try to guess why they were named that way. While common names are a lot more unpredictable than the official IUPAC rules, there are some common themes that still get involved.

To take a simple example, the following molecules are named acetylene, and acetate, respectively. Notice a trend: these are all two-carbon molecules with some functional group coming off the end. In that vein, I can guess that acetonitrile will be a two-carbon molecule with some sort of nitrogen group coming off the end...and I'd be correct! Ultimately, it's silly and somewhat pointless to try to memorize the official name, common name, and molecular formulas of every single compound you'll ever use. Instead, there are two sets of tricks you can use:

1. Nature follows a pattern. Look at compounds you already know and try to figure out what rules compounds you haven't seen should follow
2. Humans didn't pick names for no reasons. Look at the names and see if there's a common pattern.

It's not easy, but if you work hard at it for a year or two, you should have enough knowledge to piece together information about most of the compounds you'll hear about.