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Topic: Chemistry Unit  (Read 7524 times)

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Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #15 on: December 12, 2017, 03:58:59 PM »
But I have a question again here: why do we take 0,2 time 2 and not 0,1 ? I mean, it says that 0,1 mol of NaOH react, or not?

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #16 on: December 12, 2017, 05:44:46 PM »
Well, I think I got it now: we got 0,2 mole of diprotic acid, so twice the amount of NaOH.

If you do the math: 0,2/0,1=2

2*0,1=0,2

0,2* 2 (since we got two moles of NaOH)= 0,4.


right?

Offline DrCMS

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Re: Chemistry Unit
« Reply #17 on: December 13, 2017, 03:57:16 AM »
Well, I think I got it now: we got 0,2 mole of diprotic acid,
right?

No, you have 30cm3 of a 0.2mol per dm3 solution.  How many moles is that?

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #18 on: December 13, 2017, 04:53:29 AM »
6 moles?

Offline DrCMS

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Re: Chemistry Unit
« Reply #19 on: December 13, 2017, 06:43:19 AM »
No.  Stop randomly guessing.

What is 30cm3 equal to in dm3?

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #20 on: December 13, 2017, 07:04:42 AM »
3 dm^3

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #21 on: December 13, 2017, 07:19:47 AM »
so we got 3*0,2 moles of the acid...

since we got two molecules NaOH, we need 1,2 moles of NaOH.
12/0,1= we need 12 dm^3 (or 120 cm^3) of NaOH right?

Offline DrCMS

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Re: Chemistry Unit
« Reply #22 on: December 13, 2017, 07:36:20 AM »
Yes 30cm = 3dm but 30cm3 ≠3dm3

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #23 on: December 13, 2017, 07:39:46 AM »
0,03 dm^3 ...

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #24 on: December 13, 2017, 07:51:18 AM »
But my last post was basically ok, right?

Offline peterschmidt3943

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Re: Chemistry Unit
« Reply #25 on: December 13, 2017, 02:58:03 PM »
by the way: the solutions are also stated in cm^3...

so, I think that one can get to the soultion as follwoing:

30*0,2=6moles are used of the acid.

6/0,1=60*2=120cm^3 are needed... right ?

Offline sjb

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Re: Chemistry Unit
« Reply #26 on: December 13, 2017, 03:52:09 PM »
by the way: the solutions are also stated in cm^3...

so, I think that one can get to the soultion as follwoing:

30*0,2=6moles are used of the acid.

6/0,1=60*2=120cm^3 are needed... right ?

Maybe, but your error cancels. What does mol dm–3 mean? How would you answer if one volume was needed in dm3 and the given was in cm3?

Offline Enthalpy

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Re: Chemistry Unit
« Reply #27 on: December 13, 2017, 07:15:23 PM »
mol×dm–3 means mol/dm3.
For units as for other exponentials, a negative exponent means "divide".

By the way, the ISO recommends this use, rather than the divide "/" sign which is sometimes meant to apply to several units that follow.
Like J/(kg×K) is sometimes erroneously written J/kg×K while J×kg-1×K-1 is unequivocal.

Offline sjb

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Re: Chemistry Unit
« Reply #28 on: December 14, 2017, 03:51:11 AM »
mol×dm–3 means mol/dm3.
For units as for other exponentials, a negative exponent means "divide".

By the way, the ISO recommends this use, rather than the divide "/" sign which is sometimes meant to apply to several units that follow.
Like J/(kg×K) is sometimes erroneously written J/kg×K while J×kg-1×K-1 is unequivocal.

True, but not necessarily appropriate to the question in hand.

Offline Enthalpy

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Re: Chemistry Unit
« Reply #29 on: December 14, 2017, 08:38:22 AM »
Yes. I had misinterpreted this:
how can we translate dm^-3 into other units ? like, for example, in cm^3 or m^3?

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