Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: peterschmidt3943 on December 12, 2017, 12:10:59 PM
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Hello guys,
I have a question regarding the units of the following question:
What volume of a 0.10 mol dm–3 solution of NaOH is needed to neutralise 30 cm3 of a
0.20 mol dm–3 aqueous solution of a diprotic acid?
how can we translate dm^-3 into other units ? like, for example, in cm^3 or m^3?
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What do the units dm, cm, and for that matter, m stand for?
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decimetre, cenitmetre, metre...
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Ok, and a quick question regarding to the substance of the question:
a diprotic acid consists of the formula H_2 X ... why do I have to multiply 0,2mol dm^-3 times 2 ? I mean the question explicitly states that 0,2 mole OF sth... isnt the diprotic acid in total 0,2 mole dm^-3?
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Ok, and a quick question regarding to the substance of the question:
a diprotic acid consists of the formula H_2 X ... why do I have to multiply 0,2mol dm^-3 times 2 ? I mean the question explicitly states that 0,2 mole OF sth... isnt the diprotic acid in total 0,2 mole dm^-3?
Yes, but what else is in your question. What is happening in your reaction?
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NaOH gets used up...
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Do you mean we need 4x NaOH because of the fact that we got a diprotic acid?
2*0,2=0,4
0,4/0,1=4
4*30=120...
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WHat is the NaOH doing? What is it "used up" by? Why does a "di" protic acid seem to "use up" twice the NaOH?
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That I do not know...
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It acts as a base, but I dont know why twice the amount is reacting...
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I hope you don't mind us answering your questions with more questions, but that's what we do here. We try to help you learn to help yourself. Its part of our Forum Rules{click} (http://www.chemicalforums.com/index.php?topic=65859.0). If you're still stuck, why not write out the whole problem, including what you've learned in this thread, all at one to see it all together.
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Well, I know that twice the amount of NaOH should react, and that would explain the solution (120cm^3)... but I don't know why twice the amount of NaOH reacts..
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Start with the reaction equation.
http://www.titrations.info/titration-calculation
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2NaOH + H2SO4 → Na2SO4 + 2H2O... (for example)
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the mole ratio is 2:1, this explains it, right ?
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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?
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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?
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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?
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6 moles?
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No. Stop randomly guessing.
What is 30cm3 equal to in dm3?
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3 dm^3
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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?
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Yes 30cm = 3dm but 30cm3 ≠3dm3
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0,03 dm^3 ...
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But my last post was basically ok, right?
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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 ?
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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?
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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.
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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.
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Yes. I had misinterpreted this:
how can we translate dm^-3 into other units ? like, for example, in cm^3 or m^3?