Formula Homework

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

You're absolutely right. In principle, we're supposed to have formalnamingsystems (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.

Good luckk!

I need help to calculate the quantity of product needed from a formula
Let met explain, i have the following equation : $\ce{KOH(aq) + HCl(aq) → KCl(s) + H2O(l)}$
In this equation we combine :
- 1 mole of $\ce{KOH}$ (potassium hydroxide)
- 1 mole of $\ce{HCl}$ (hydrochloride acid)
Am i right ?

If yes, so let's do the math
To calculate the quantity of matter (in g) needed for the following equation i use this formula:


Let's do this for the $\ce{KOH}$
Wikipedia say's that $\ce{KOH}$ molar mass is : $M = 56.6$ g/mol
So aplying the formula would give : $m = 1 * 56.6 = 56.6$g

Let's do this for the $\ce{HCl}$
Wikipedia say's that $\ce{HCl}$ molar mass is : $M = 36,461$ g/mol
So aplying the formula would give : $m = 1 * 36,5 = 36.5$g

Now in the products size i choose the $\ce{KCl}$ for example
Wikipedia say's that $\ce{KCl}$ molar mass is : $M = 74,55$ g/mol
So aplying the formula would give : $m = 1 * 74,55 = 74,55$g

So if i'm right, that's mean that i'll need 56.6g of $\ce{KOH}$ and 36.5g of $\ce{HCl}$ in order to produce 74,55g of $\ce{KCl}$ ? Or am i doing someting wrong ?


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