Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: chris p on July 18, 2005, 03:58:10 PM
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Find the PH of NaNO2 if the Ka of HNO2 is .00045
I attempted the problem and obtained 12.17. Can someone please try to do the problem and let me know what they get? Thanks in advance.
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I just realized... You can't calculate pH without known concentration of salt. The question as you posted it (here and elsewhere) has no solution. Sorry I haven't noticed it before.
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Is there anyway to calculate the concentration (or Molarity) of the salt using the Ka?
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we have to make assumption regarding the concentration of the salt.
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So to answer your question no. Concentration is never related to the Ka of a substance. The pH of a solution is, by definition, the concentration of H3O+ (or,e quivalently, H+), which is determined by the ceoncentration of acidic/basic substance you started with.
Another example: HCl is hydrochloric acid. At a concentration of 0 M, the pH of water is 7. When you have water with 1 M HCl, your pH is -log(1) = 0. Quite the difference, just because of the concentration eh? And the Ka certainly doesn't change. It's a constant (for a given set of conditions i.e. temperature)
Hope this helps :)
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Since there is not set way to calculate the concentration and if I use .5 randomly as the concentration, will my answer of 12.17 ph be correct? Thanks.
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Your answer is a way too high. Show your math.
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I tried again.
x^2/.5=Kb
-log OH = pOH
14-pOH = 8.5
Is this answer better?
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Much better :)
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let [NO2-]=x,
NO2- + H2O => HNO2 +OH-
initial [] x 0 0
change [] -y y y
final [] x-y y y
Kb=Kw/Ka=2.2E-11=(y^2)/(x-y)
y^2 + (2.2E-11)y - (2.2E-11)x = 0
y=(-2.2E-11 + sqr(2.2E-22 - 8.8E-11x))/2
x is the concentration of NO2-, which should be provided in the question.
plug in the concentration for x, solve for y, the concentration for OH-.
then, pH=14+log(y)
if you plug in 0.5 for x, pH is 8.52
if pH is 12.17, then x must be 9844927.323...
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So to answer your question no. Concentration is never related to the Ka of a substance. The pH of a solution is, by definition, the concentration of H3O+ (or,e quivalently, H+), which is determined by the ceoncentration of acidic/basic substance you started with.
Another example: HCl is hydrochloric acid. At a concentration of 0 M, the pH of water is 7. When you have water with 1 M HCl, your pH is -log(1) = 0. Quite the difference, just because of the concentration eh? And the Ka certainly doesn't change. It's a constant (for a given set of conditions i.e. temperature)
Hope this helps :)
For pH is 0 for HCl 1 M....I think the calculation is like this..
[HCl] = 1 M
[H+] = 1 M
pH = -log 1 e 0
= 1 - log1 = 1 - 0 = 1
Am I right??
Regards,
Victor
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no. 10^what = 1 --> a power can only be 1 when the power equals 0. Thus log 1 x 10^0 = 0 and thus -log 1 x 10^0 = 0
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a power can only be 1 when the power equals 0
0 = 1
Interesting result ;)
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0 = 1
Interesting result ;)
i mean x^0 is always 1 ;) sorry for the bad english. I meant the exponent ;)
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Um, actually in my school I get a little bit confused with this...
can pH or pOH be in minus?
if can't please take a look at this question.
count the pH of 100 ml HCl 3M!
If I cont it in my way, it would be like this.
[HCl] = 3 M
[H+] = 1 x 3 M = 3 M
pH = -log 3 = (minus mark)!!
Please *delete me*!
Regards,
Victor
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can pH or pOH be in minus?
Yes.