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Topic: pH of nitric acid  (Read 15521 times)

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Offline Wil"

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pH of nitric acid
« on: March 18, 2009, 06:34:54 AM »
I want to ask what's the pH of a 1x10-8M nitric acid(aq)?

I know how to calculate the pH,
pH = -log[HNO3]
which gives you pH=8 in this case.

But nitric acid is an acid, isn't it?? How its greater than 7??

I checked wiki before and I got "nitric acid" doesn't donate H+

So what's so dfferent of HNO3??

Can  anyone tell me more?? I just couldn't find it....

Thanks!
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Offline sjb

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Re: pH of nitric acid
« Reply #1 on: March 18, 2009, 07:35:48 AM »
Quote from: Wil" on March 18, 2009, 06:34:54 AM
I know how to calculate the pH,
pH = -log[HNO3]

No, the definition of pH is -log[H+].

Try again, with this revised information.
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Offline Wil"

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Re: pH of nitric acid
« Reply #2 on: March 18, 2009, 10:21:02 AM »
but the value won't make any difference, because 1x10-8M HNO3 gives 1x10-8M H3O+

still I got pH=8....
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Offline sjb

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Re: pH of nitric acid
« Reply #3 on: March 18, 2009, 10:27:10 AM »
Why are you concerned that the pH is greater than 7?

What solvent are you measuring the pH in?
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Offline AWK

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Re: pH of nitric acid
« Reply #4 on: March 18, 2009, 10:52:22 AM »
Take into account also hydrogen ions from water, and solve quadratic equation.
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Offline Wil"

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Re: pH of nitric acid
« Reply #5 on: March 18, 2009, 11:14:31 AM »
Quote from: AWK on March 18, 2009, 10:52:22 AM
Take into account also hydrogen ions from water, and solve quadratic equation.

but why so sudden I have to take the H+ from water into account??

I got it 6.95....
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Offline Borek

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Re: pH of nitric acid
« Reply #6 on: March 18, 2009, 11:21:01 AM »
Generally speaking you should take them into account always, however, for more concentrated acids they can be safely ignored - their concentraion is just too low to play any role. That's not th ecase when their concentration becomes comparable with concentration of acid.

6.95 doesn't look correct to me.

http://www.chembuddy.com/?left=pH-calculation&right=pH-strong-acid-base
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Offline Loyal

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Re: pH of nitric acid
« Reply #7 on: March 19, 2009, 02:46:38 AM »
Quote from: Wil" on March 18, 2009, 11:14:31 AM
Quote from: AWK on March 18, 2009, 10:52:22 AM
Take into account also hydrogen ions from water, and solve quadratic equation.

but why so sudden I have to take the H+ from water into account??

I got it 6.95....

When you are dealing with any acid concentration that is around 10-5M or lower the H+ produced from water is enough to make an impact or in the case of 10-7M or lower the water actually produces as much or more H+.   
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