Assume diatomic molecule or not on real MCAT???

[GeorgianCMV]

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I thought that you should always assume diatomic. But Kaplan has certain questions where if they just say "oxygen" rather than "diatomic oxygen" and don't mention "standard state" or anything, you assume just O, not O2. Should I ALWAYS assume diatomic for the real MCAT when doing stoich calculations?

 

[Kaustikos]

I thought that you should always assume diatomic. But Kaplan has certain questions where if they just say "oxygen" rather than "diatomic oxygen" and don't mention "standard state" or anything, you assume just O, not O2. Should I ALWAYS assume diatomic for the real MCAT when doing stoich calculations?

Yes, in that case. Those elements are diatomic. I guess Kaplan is trying to make you remember that although it is O2, oxygen can be by itself?

 

[physics junkie]

I'm sure it will be very clear when you take it what they want you to do.

 

[BloodySurgeon]

well if they are talking about oxygen they are talking about the element oxygen. But when they talk about molecular oxygen or a mole of oxygen, always assume O2. For calculations (unless stated otherwise), I can't think of a situation when you would not use O2 like a mole of oxygen (36g/mol).

*thought of one... lol... if you have have a ratio of 12g C, 2g H, and 16g O... the empirical formula is what?

answer--> CH2O

 

[nohatehere]

Major bump..
Sorry, but I'm still a bit confused in when we apply the diatomic rule and elements in natural states..do we only use it when Oxygen (and the other 6 diatomics) is stated as a GAS in the problem?

Bloody Surgeon: do you mind if you clarify a little bit on your explanation? 1 mole of Oxygen, etc.

a practice problem I came across:
How much calcium metal combines with one gram of oxygen to form CaO?
In this case, Oxygen is only one mole (in ratio with 1 mole of Ca), only 16g applies? and because I think CaO is NOT in gas form?
(By the way, the solution used 16g instead of 32g, which seems to actually conflict with Bloody Surgeon's explanation, I think..)

Thanks

 

[Ttan]

Im pretty sure whenever you are looking at oxygen by itself as a reactant in a reaction, it will always be o2. It doesnt matter what its phase (phase=solid liquid gas or SCF) is either, when it is alone its going to bind with itself and be o2.

Balanced Reaction: 2Ca(s)+O2(g)--->2CaO(s) Plug in your O2 mass, factor in mole ratios, and compute

 

[RogueUnicorn]

Im pretty sure whenever you are looking at oxygen by itself as a reactant in a reaction, it will always be o2. It doesnt matter what its phase (phase=solid liquid gas or SCF) is either, when it is alone its going to bind with itself and be o2.

Balanced Reaction: 2Ca(s)+O2(g)--->2CaO(s) Plug in your O2 mass, factor in mole ratios, and compute

yes as long as it doesn't come from something else first.

 

[chemtopper]

To be very clear about it ---
whenever you are dealing with a reaction than oxygen is always O2 .If you are not doing any calculations based on reactions than just take care than oxygen is O and oxygen gas is O2
Like calculate how many moles of Oxygen are there in 20 gm of oxygen gas ?
how many moles of Oxygen molecules (or gas)are there in 20 gm of oxygen gas ?
I hope I have made things a bit clear now.