[Licensed Criminal]

Get it? It's a pun on the movie 'Star Wars: The Empire Strikes Back'.

Yes?




...No?  Damn. I spent ages thinking of that...

Err... anyway, I'm in need of your infinite wisdom. I've got a question on calculating empirical formulae, and I need to know that what I'm doing is correct. It goes like this...

'Work out the empirical formula of the compound with the following percentage compositions by mass:

27.4% Sodium, 1.2% hydrogen, 14.3% carbon, and 57.1% oxygen.'

I got the answer Na⁺(COH)^-, which is probably incorrect. Could you show me the stages involved in finding the answer to this question, with info on the ratios?

[enahs]

Yes, it is incorrect.

Step 1, assume you have a 100g sample of the compound.
This now means you know you have 1.2g H, 14.3g C, 27.4 g Na and 57.1g O.
Determine how many moles that relates to for each compound, and then determine the molar ration of each.

Do not try and bother making it be a legitimate structure, this is the empirical formula. That means it is the smallest integer number ratio of the compounds, not what the actual compound is.

[refid]

H 1.2g / 1.01(g/mol)  =1.188
C 14.3g  / 12.01(g/mol) =1.191
Na 27.4g  / 22.99(g/mol) = 1.192
O 57.1g  / 16(g/mol)      = 3.569

Smallest ratio H 1.188

H 1.188/1.188=1.0
C 1.191/1.188=1.0
Na 1.192/1.188=1.0
O 3.569/1.188=3.0

Na+ [CO3H]-
This what I got..Na+ [CO3H]-  did i do something wrong?

[Licensed Criminal]

Thanks enahs and refid! I tried it again last night, and got the following answer (which is the same as refid's):



Is it correct, then...?

[enahs]

Correct and Correct.

[sjb]

The only thing I'd query is how you can tell *from the empirical formula* that it's ionized...

S

[hmx9123]

Possibly through the experience of dealing with baking soda? (NaHCO₃)

[sjb]

Possibly through the experience of dealing with baking soda? (NaHCO₃)

I don't deny that Na⁺HCO₃^- is the correct formula, but that requires further knowledge.

How about if the maths worked out to give an empirical formula of C₄H₁₂N₃O₆SiP?

Would P(CH₃)₄⁺ Si(NO₂)₃^- be an acceptable answer? (hypothetical compound, not sure if it exists )

S

[enahs]

Would P(CH₃)₄⁺ Si(NO₂)₃^- be an acceptable answer? (hypothetical compound, not sure if it exists )

The answer is, many times you can not from the empirical formula. The empirical formula is just one of many tools we use to elucidate the structure of molecules.