Quick Question About Empirical Formulas in Chemistry

[HemlockGrey]

I realize I should probably know this, but I'm calculating an empirical formula and I end up with some weird fraction, like 2.5 0 and 1 P, what do I do with that? Is it P02, P03, or is it P205? And if it's like 2.8 O 1.2 P I can just round, right?

[Exonerate]

My chem is rusty, but IIRC all the empirical formula gives you is the ratio... In this case, you have P2O5 - you can't have half an atom, so you'll have to find a molecule where the ratio in the empirical formula is the same without fractions of atoms.

Sorry if that was hard to understand, it's been over a year since I took chem.

[Lord Sabre Ace]

I realize I should probably know this, but I'm calculating an empirical formula and I end up with some weird fraction, like 2.5 0 and 1 P, what do I do with that? Is it P02, P03, or is it P205? And if it's like 2.8 O 1.2 P I can just round, right?

The empirical furmula needs to have integers. I think I remember my Chem teacher telling us that if you get within .1 of a whole number you can round or something like that.

[Lord Sabre Ace]

edit:

should be formula, not furmula

[tharkûn]

I realize I should probably know this, but I'm calculating an empirical formula and I end up with some weird fraction, like 2.5 0 and 1 P, what do I do with that? Is it P02, P03, or is it P205? And if it's like 2.8 O 1.2 P I can just round, right?

P2O5, depending on context, P4O10 makes sense as well (and is used in some literature).

Can you round? Depends on what you are doing. The quick way to work out the numbers is to multiply by two, then three, then four, etc. until all are whole numbers - check the resulting compounds against the literature (either using a CRC, Merck, or internet search) and see what makes sense - that will get you through all lower level chem 95% of the time or more rounding is pretty much when it is "close enough" it can't be anything else reasonable.

For example let's say you burnt phosphorus in a bomb with excess oxygen and calculated this value back. Does this make sense? Well phosphorus pentoxide is the normal combustion production and its toxic nature would be consistent with the lab procedures required for burning it in a bomb. Likewise if you were working out a book problem for sugar purification this would make sense. On the hand if were calculating the formula of a polyatomic ion containing both phosphorus and oxygen derived from an acid this wouldn't make sense.

When you start dealing with doping, "defected solids", and some of the other upper level stuff thou shalt not round; it is entirely acceptable to have screwy things like Na2.8Zr6.5Si2O17.8. When you get to those cases you have a few tricks to see if the formula is sensible, but you should be able to trust your practical measurements (or do them over or being doing something screwy enough nobody can tell at the present time).

The empirical furmula needs to have integers. I think I remember my Chem teacher telling us that if you get within .1 of a whole number you can round or something like that.

Depends on the context. Low level stuff, more or less correct. Upper level stuff you can have numbers far smaller than .1; it really is the only sensible way to include dopants in the formula; likewise when you have substitutable ions formulas with subscripts of x and (n-x); n being a whole number are quite easy to prepare for any given value of x.

[Zero]

P2O5, depending on context, P4O10 makes sense as well (and is used in some literature).

I thought that the empiracle formula was supposed to be the lowest whole-number ratio thingy you could find, so that P4O10 would be P205, or something like C12H14 would be C6H7. The molecular formula can be different, and a different molecular formula will exhibit different properties, but I'm pretty sure the empiracle formula is supposed to be the lowest whole number thingies you can find.

I just had a test on this, but I got a C, so...

[BlkbrryTheGreat]

I realize I should probably know this, but I'm calculating an empirical formula and I end up with some weird fraction, like 2.5 0 and 1 P, what do I do with that? Is it P02, P03, or is it P205? And if it's like 2.8 O 1.2 P I can just round, right?

An Empirical Forumla is the simplest whole number ratio of atoms within a compound. So no, you cannot round since it would change the ratio of atoms within the compound. Assuming your calcuations are correct, what you would need to do would be to multiply both sides of the equation by the number that will give you the lowest whole numbers for all the molecules present within the equation. No fractions or whole numbers are allowed in an empirical forumla.

[Phil Skayhan]

For the example you gave, I'm assuming this is after you have determined the number of moles. You could convert the 2.5 to its fractional equivalent. So you would have:

(5/2 oxygen)/(1 phos)= P2O5

[tharkûn]

I thought that the empiracle formula was supposed to be the lowest whole-number ratio thingy you could find, so that P4O10 would be P205, or something like C12H14 would be C6H7.

Depending on what you are reacting with P4O10 might make sense because you have another atom (or complex) it coordinates to. If you are talking in the context of hydration or something else sometimes the molecule of interest is going to show up as a multiple.

The molecular formula can be different, and a different molecular formula will exhibit different properties, but I'm pretty sure the empiracle formula is supposed to be the lowest whole number thingies you can find.

It depends. If you are working with pure phosphorus pentoxide then you would correctly use the P2O5, if you are working from a substance where you have M.P4O10 in your unit cell then you would get P4O10 out of the math. I know not what starting material was used or if chelation, hydration, or something else was part of the problem. In the vast majority of lower level cases you will see P2O5, but there are cases where it makes sense to talk about an empirical formula of P4O10. Mainly I mentioned it as a CYA statement just in case this wasn't the simple one.

For more difficult stuff, like that sodium zirconium crystal, you will only get whole numbers by carrying around an idiotic number of atoms. For doping you really don't want to go to whole numbers for 100 ppm.