[kennnho]

Okay the question goes like this, first they give you the equation of a chemical reaction

Na₂O + H₂O  2NAOH + H₂

6.2 g of sodium oxide is added into 500cm cube of water. what is the concentration of the solution produced?

So i did my calculations and the answer i got is 0.4mol/dm cube
when my teacher gave back the papers she disccused the question with us and kept on insisting that the correct answer is 0.2mol/dm cube.
she said to calculate the concentration you just have to find the number of mol of sodium oxide used, which is 0.1 mol and divide it by the volume of water, which in this case is 500cm cube(0.5 dm cube) , and you get 0.2mol/ dm cube
now what i don't get is, isn't NAOH supposed to be used to calculate the concentration and not sodium oxide, since NAOH is the solution produced? since 1 mol of sodium oxide will produce 2 mols of NAOH, 0.1 mol will produce 0.2 mol, then to calculate the concentration you take 0.2 and divide it by 0.5 dm cube and you get 0.4mol/dm cube.

I'm kinda confused myself, so to summarize, since the the formula to calculate molarity / concentration is solute divided by solvent, which is the solute in this case, sodium oxide or NAOH?
Any form of answers will be much appreciated, and sorry if i've made some wrong assumptions about anything, just trying to learn here, thanks

[Borek]

Sometimes it may be convenient to express concentration (composition) in terms of some particular compound, even if it technically doesn't exist in the mixture. It is quite popular to express composition of fertilizers in terms of percentage of K₂O, P₂O₅ and N₂, even if neither of these compound is present. So, for some applications it can be convenient to say solution is 0.2 M in Na₂O, for other applications it can be more convenient to say it is 0.4 M in NaOH. As long as there is no ambiguity about which way of expressing concentration you use, and as long as it is accepted way of expressing the concentration in the community you are dealing with (and different groups will use different ways of expressing composition) everything is OK.

That being said, I don't remember ever seeing concentration of Na₂O in water. But the meaning is obvious. OTOH question - as posted - is ambiguous, as it doesn't say "concentration of what".

[kennnho]

oh, i think i get it now, so to make it clear, what you're saying is 0.2mol/dm cube Na₂O is actually equal to 0.4mol/dm cube NAOH? and the problem here is the question doesn't state in what terms do they want it, NAOH or Na₂O, so technically, both answers are correct, am i right?
thanks alot, that really helped
and also for the extra information, pretty much widened my understanding on the subject.

[Borek]

so technically, both answers are correct, am i right?

Yes.

and also for the extra information, pretty much widened my understanding on the subject.

Some things don't make sense without a context, that's why I added the extras. Glad you liked it 

[magician4]

maybe, with all due respect to the honorable Gentlemen, somebody should have pointed out that the initial reaction equation shown is UTTERLY , UTTERLY WRONG

having said this, I additionally would like to point out that the pK_b of O^2- is somewhere in the ballpark of minus 15 to minus 20 *⁾
... so for all practical purposes, in aqueous solutions it is considered to be inexistant
hence, technically, your answer insofar should be: [O^2-] < limit of detection  [itex] \approx [/itex] 0


regards

Ingo


*⁾
meaning that, at pH 13,25 you wouldn't have more than a few thousands of named oxygen ions in 1 L of water (if we'd talk pKb -20, for example)
this makes the concentration of this stuff even lower than the concentration of ubiquity of even rare elements in general, and you'll meet more of those in a liter of even the purest water (i.e. have more gold, silver, platinum ... present) than oxygen dianions
... maybe with the exception of francium**⁾  and thatlike , i'll give you that

**⁾ it's been calculated, that in general at any given moment in time naturally there are no more than 4 atoms of francium present in the whole of world's oceans
that's less than oxygen dianions, I agree

[kennnho]

Borek:
once again, thank you so much

Magician4:
No offence, but i really have no idea what you've just said. i tried reading it over and over but i don't understand most of it, maybe because i've yet to learn it in school yet, sorry  could you explain it in simpler terms? thanks

*p.s. i noticed the equation is wrong, it's not supposed to have hydrogen gas as a product, only NAOH, and i would like to add that the question stated above, everything from the equation to the question itself, is exactly the same as what was printed on the question paper for my examination, so i guess my teacher made a mistake while writing the equation. But it doesn't really matter now since i can't change my marks, i just wanted an answer for this question as it has been troubling me for quite some time. thank you all for your time, i really appreciate it.

[magician4]

*p.s. i noticed the equation is wrong, it's not supposed to have hydrogen gas as a product, only NAOH, and i would like to add that the question stated above, everything from the equation to the question itself, is exactly the same as what was printed on the question paper for my examination, so i guess my teacher made a mistake while writing the equation.

If you did notice it , why did you post the wrong equation here in first place? ... (just asking myself)

(...) could you explain it in simpler terms? thanks

brief version, talking "school level":

-  there is no such thing as O^2- ions dissolved in water

- O^2- in contact with water instantaneously and violently will react to give OH^- :
O^2- + H₂O  2 OH^-

- therefore , the concentration of O^2- in water safely could be called "zero for all practical purposes"
(everything else is purely academical discussion and irrelevant for school)

- O^2- ions dissolved in water plain and simply are inexistant, and any equation stating otherwise is false

regards

Ingo

[Borek]

there is no such thing as O^2- ions dissolved in water

And how is that related to the original question?

As I explained earlier, there are many ways of expressing composition and concentration, they often don't refer to the actual compounds being present. Kjeldahl method gives nitrogen as ammonia - even is solutions/compounds that never contained any ammonia. Calcium phosphate doesn't contain P₂O₅, yet telling someone that it contains 46% of P as P₂O₅ is perfectly OK. Na₂O as a way of telling composition of water solution is unusual, but as long as it is clearly stated what you mean, it is perfectly unambiguous.

[magician4]

And how is that related to the original question?

in my opinion, this is related to the original question insofar, as this most incompetent teacher of @kennnho insisted in that:

(...)when my teacher gave back the papers she discussed the question with us and kept on insisting that the correct answer is 0.2mol/dm cube.(...)

(most probably she's the same genius that originally invented this horrible piece of equation " Na2O + H2O  2NAOH + H2 " )

and no, I don't support the idea that the way to calculate / define  things in chemistry is kind of "personal gusto" , esp. not if @kennnho IS right and the teacher IS wrong

therefore, @kennnho deserves to be supported in his correct understanding of the what has happened - at least here in the forum

regards

Ingo

[kennnho]

If you did notice it , why did you post the wrong equation here in first place? ... (just asking myself)

i posted the wrong one because on my question paper it does include the H₂ gas as a product, what i'm doing is merely copying the question and pasting it to show everyone that this was the exact question i stumbled upon during my examinations. I was afraid that i would be wrong somehow so i decided to include the H₂ as well because i was afraid that by altering the original question it might alter the results as well. so to put it simply, i noticed there was something wrong with the equation, but i was not 100% sure, so i posted it without changing anything from the equation to the question itself. My apologies for not making this clear in my first post.

and thank you for your time, i think i kinda understand now, so what you're saying is there's no such thing as O^2- ion in water because it reacts with water to give OH^-, so saying 0.2mol/dm cube in Na₂O is wrong?

Spoke to my teacher about this today, she insisted that i was over thinking  and all I have to do is follow the formulae and use mass of solute over volume of water, asked me to stop talking about it while i was explaining, oh well  at least i know the truth.

Borek:
i understand what you meant, but it's really hard explaining to a teacher, especially when you're just a normal student. but really thank you, i might not get the mark my chemistry paper, but at least I learnt something, and that to me, is truly what education is about.

[magician4]

so what you're saying is there's no such thing as O^2- ion in water because it reacts with water to give OH^-, so saying 0.2mol/dm cube in Na₂O is wrong?

that's exactly my point , yes

... and it is very important, in my opinion, for pupils to learn this and apply this (i.e. that water is "reactive" to some substances / structures, and will destroy / transform them, so they're inexistant there). We're taking a lot of time teaching that for example very strong bases ( like O^2-, CH₃^- , NH₂^- , CH₃CH₂O^- ...) or acids ( like HCl , H₂SO₄ ... )   though in principal existand and well known under completely different circumstances, are not existant in water and can be safely neglected there.

so, whenever you wish to express that, for example, you have a waterbased HCl where the "hypothetically undissociated" concentration of named acid was 0.1 mole/L , you would write c₀(HCl) = 0.1 mole/L , keeping in mind that the real concentration of HCl c(HCl_{undisssociated}) in this solution [ i.e. c(HCL) ] is essentially 0 mole/L , whereas new species like Cl^-_aq. and H⁺_aq. did appear instead:
c₀(HCl) = 0.1 mole/L  c(HCl) = 0 mole/L ; c(Cl^-)aq. =0.1 mole/L ; c(H⁺)aq. = 0.1 mole/L

(...) she insisted that i was over thinking  and all I have to do is follow the formulae (...)

[irony on] yeah, that's exactly what we're sending children to school for: to learn stop thinking and become robots instead [/ irony off]

to repeat: you were right, she was wrong

to start with, she did give you the false equation. Furthermore, she now doesn't have the guts to admit that she was in error with respect to "concentrations", too, and instead orders you to stop thinking in a for real scientific way

that's what in my opinion is a very, very bad teacher

oh well    at least i know the truth.

that, by the end of the day, is the important part, and I kindly invite you to come back here and discuss your thoughts often, esp. whenever you're in doubt


... and yes, it is an esp. disturbing situation to have to follow oversimplified schemes, knowing that at times they are wrong - just for not endangering your grades

good luck


regards

Ingo

[kennnho]

thank you so much @magician4 for supporting me and giving me a better understanding of what's going on, i will be back here whenever i have questions regarding chemistry, thanks