How to tell when to have diatomic element or not?

[Sammy1024]

How do you tell when you're supposed to use the diatomic version of an element? I know there's 7 of them, but not when to use a regular version or diatomic version.

Like if you're doing stoichiometry and it says oxygen, then do you do 1 mol O = 16 g or 32 g?

Is there certain instances when you use diatomic vs. regular?

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[Teleologist]

Have No Fear Of Ice Cold Beer

Hydrogen
Nitrogen
Fluorine
Oxygen
Iodine
Chlorine
Bromine

Are all found as diatomic species at standard state conditions.

 

[Teleologist]

Like if you're doing stoichiometry and it says oxygen, then do you do 1 mol O = 16 g or 32 g?

That depends. Are you working with O or O2?

 

[Sammy1024]

Like if a question says oxygen, how do I know if i'm supposed to refer to O or O2?
Is there a way to tell?

 

[Teleologist]

Like if a question says oxygen, how do I know if i'm supposed to refer to O or O2?
Is there a way to tell?

Yes. Can you give an example though?

 

[Sammy1024]

Ummm, making one up, but something like...

There is a container with 28 g Nitrogen and 32 g Oxygen, what is the partial pressure of N in a 6 atm container?
Or
There is oxygen in a container with a partial pressure of 4 atm. How many grams of Oxygen is that?

Like there's nothing about STP, or anything so how do I know whether to convert oxygen to moles by using 1 mole is 32g or 1 mole is 16 g?

 

[Teleologist]

STP isn't the same as standard state conditions. In your problem I think it'll be safe to think that you're working with O2 not O. Sure, if you heat up the reaction vessel high enough, you'll have O as well as O2, but I don't think the test will try to get you like that.

 

[Sammy1024]

I think i did have a question similar to that, but I used 16 g = 1 mole and it was right. So if it doesn't say anything should i assume STP?

 

[Teleologist]

I think i did have a question similar to that, but I used 16 g = 1 mole and it was right. So if it doesn't say anything should i assume STP?

I don't like to assume. Can you show me the actual question?

 

[Sammy1024]

If a flask containing 0.28 g of nitrogen and 0.64 g of oxygen has a total pressure of 3 atm, what is the partial pressure of nitrogen in the flask?

A. 0.5 atm
B. 1.0 atm
C. 2.0 atm
D. 3.0 atm

The answer was B, which is what i picked. I guessed I solved it with ratios, but the answer shows that they used O2 and N2.

 

[justadream]

@Sammy1024

Well the question is talking about gases so it's pretty easy to infer that you should be using O2 and N2.

 

[Sammy1024]

So for gasses they're always diatomic? And if there was a super simple stoichiometry problem like you have 32g oxygen, how many moles it this, you would also assume diatomic because it would most likely be in gas form?

 

[justadream]

@Sammy1024

I'm no Chemistry expert like @Teleologist but I would first look at the reaction if it's provided or if it can be inferred (for example, combustion uses O2).

In most cases, I think you should use O2 but I'm sure there are a bunch of weird scenarios in which you use O.

 

[Sammy1024]

I guess i'm just trying to figure out when I wouldn't use O2 and instead use O.

So basically if the reaction says that it's O and then possibly if oxygen is in non-gaseous form?

 

[justadream]

@Sammy1024

If the equation says it's O, use O.

For example, I just found some random reaction:
O2 + ℎν → 2O
O + O2 → O3
http://en.wikipedia.org/wiki/Ozone-oxygen_cycle

So here if you are going to find the grams of product in the first reaction, you'd use 16g and not 32g.

But I don't know for sure if not being in O2-form means it is non-gaseous.

 

[Sammy1024]

Basically, try to always use O2 unless it says so otherwise.
hopefully I don't have to make that decision on the test!

Thank you for all your help! I'm sure I made this crazy complicated!

 

[type12]

@Sammy1024

I'm no Chemistry expert like @Teleologist but I would first look at the reaction if it's provided or if it can be inferred (for example, combustion uses O2).

In most cases, I think you should use O2 but I'm sure there are a bunch of weird scenarios in which you use O.

I lol'ed.

Anyway, yep, @justadream is correct; and you'll get familiar and comfortable with the diatomic elements through practice. After enough practice, it should just "click."