Fundamental F. 17

[Stella Nguyen 1J]

Hi Everyone!

I have a quick question about problems that are similar to this. After I find the empirical formula and molecular formula, how would I know which order each element comes in. I got the correct ratio in the empirical formula and subscripts in the molecular formula, however I got the order of the elements incorrect.

Problem: Osmium forms a molecular compound with mass percentage composition 15.89% C, 21.18% O, and 62.93% Os.
(a) What is the empirical formula of this compound? (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g⋅mol^-1. What is its molecular formula?

Thank you!

[Leyna Tran 2D]

I was also wondering the same thing! I was able to solve for the empirical formula, but was unsure of the order of the elements. Will it count against us on a midterm or final if we don't know the correct order to write the elements? Does it matter?

[Lily Kiamanesh 2G]

Hi! I believe that in general, the order is carbon first, then hydrogen, then the other elements ordered alphabetically, but there are some exceptions to this rule. I think that usually, the problem will state the mass composition of the elements in the order in which they are listed in the molecule (15.89% C, 21.18% O, and 62.93% Os --> C, then O, then Os).

[Melis Kasaba 2B]

I believe the order of elements in a chemical formula follows something called the Hill System. Basically, carbon comes first, then hydrogen, and then the other elements in alphabetical order. If the formula doesn't have carbon, then all the elements, including hydrogen, are listed in alphabetical order.

[Kelly Ha 1K]

This question was brought up in my discussion section and my TA stated that the order doesn't really matter. I'm not sure if Dr. Lavelle has mentioned whether or not it will count against us on the midterm or the final but this is what I heard during my discussion section. To be safe though, I think it would be best to follow the general "Hill System" someone previously mentioned in this topic (carbon first, then hydrogen, then elements in alphabetical order), but to reiterate I don't believe it matters. Hope this helps!

[Shalyn Kelly 3H]

Hello, I do believe I heard someone - either Lavelle or I'm tripping - mention that when grading, they'll be looking to give you points (so if the balancing and everything else is correct, then I don't see them keeping points from you).
Take the equation CH₄ + 2O₂ --> CO₂ +2H₂O, if you write it as CH₄ + 2O₂ --> 2H₂O + CO₂ or H₄C + 2O₂ --> CO₂ + 2H₂O then I think you'll still technically be correct - at the most maybe a note saying "typically it's written another way" or something. However, since the person above mentioned the "Hill system" I'd definitely try and use that when possible.
I'm also glad to have learned that, because I've just been guessing and matching my equations to others I've seen as best I can lol.

[FionaHunter21]

For formulas with C,H, and O, I think they follow that order, but outside of carbon being first and hydrogen second, I've never seen a formal description of what order to write them in.