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Topic: acids/bases  (Read 8309 times)

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777888

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acids/bases
« on: January 05, 2005, 10:09:11 PM »
1.Calculate the pH and pOH of HCl(aq) prepared by dissolving 30.5kg of HCl(g) in 806L of water at SATP. State the assumptions that are made when doing this calculation.
.....
I calculated and got [H+]=1.0379mol/L and pH=-0.0162, but for the pH, the answer in the textbook is positive...how come?

Can someone help me? Thanks!
« Last Edit: January 05, 2005, 10:09:44 PM by 777888 »
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cerez05

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Re:acids/bases
« Reply #1 on: January 05, 2005, 11:33:42 PM »
pH = -log[H+]
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777888

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Re:acids/bases
« Reply #2 on: January 06, 2005, 01:01:59 AM »
Quote from: cerez05 on January 05, 2005, 11:33:42 PM
pH = -log[H+]
YES, but I get  pH=-0.0162 and this is not the answer....
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Offline AWK

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Re:acids/bases
« Reply #3 on: January 06, 2005, 01:48:59 AM »
Calculate the molar concetration properly. (pH =3)
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777888

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Re:acids/bases
« Reply #4 on: January 06, 2005, 10:21:25 AM »
1.Calculate the pH and pOH of HCl(aq) prepared by dissolving 30.5kg of HCl(g) in 806L of water at SATP. State the assumptions that are made when doing this calculation.
         100%
   HCl  ->  H+ + Cl-

n(HCl)=30500/36.46=836.5332mol
[HCl]=836.5332/806=1.03788mol/L
[H+]=1.03788mol/L
pH=-log(1.03778)
pH=-0.0161
I did it properly! But I got a negative answer! Is there something about solubility, maybe HCl(g) didn't dissolve completely? If so, how can I calculate the pH(the answer is positive at SATP)?

Thank you for your *delete me* :)
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Demotivator

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Re:acids/bases
« Reply #5 on: January 06, 2005, 12:24:34 PM »
Looks right to me.
pH can be negative at times. The requirement is only:
pH + POH = 14
If the answer is the same number, but positive, it probably missed the sign. If the answer is a different number, then I don't know why.

However, the actual PH might be positive because:
1) PH is more accurately a measure of H+ activity, not molarity (but that is an advanced topic you're probably not responsible for).
2) The volume of the solution might have expanded when HCL dissolved. But I can't see how that can be determined from the information given.
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777888

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Re:acids/bases
« Reply #6 on: January 06, 2005, 12:41:42 PM »
Quote from: Demotivator on January 06, 2005, 12:24:34 PM
Looks right to me.
pH can be negative at times. The requirement is only:
pH + POH = 14
If the answer is the same number, but positive, it probably missed the sign. If the answer is a different number, then I don't know why.

However, the actual PH might be positive because:
1) PH is more accurately a measure of H+ activity, not molarity (but that is an advanced topic you're probably not responsible for).
2) The volume of the solution might have expanded when HCL dissolved. But I can't see how that can be determined from the information given.
The answer in the book is the same but positive.
Can solutions at SATP have a negative pH? Doesn't pH have a range of 0-14?
The question also asked what assumptions I have made. I don't really know...
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Demotivator

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Re:acids/bases
« Reply #7 on: January 06, 2005, 12:49:00 PM »
There's no law that limits ph to 0-14.
kw = 10^-14=[H+][OH-]  or PH + pOH =14
That's the only law.

Assumptions are 100% disscociation and no change in volume of water or solution.
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Offline AWK

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Re:acids/bases
« Reply #8 on: January 07, 2005, 01:15:08 AM »
My fault - I did not notice kg in mass of HCL

%HCL (w/w) = (30.5/836.5)x100=3.646 %
density of HCl= 1.018
Molarity M= 10x3.646x1.018/36.46=1.018
pH=-0.008

pH of water solution for different strong acids and bases is between -1 and +15
when calculated from concentrations.
Neglecting density of solution we got eror of order 1% for molarity and 0.01 unit for pH
in this case
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AWK
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