March 29, 2024, 01:30:29 AM
Forum Rules: Read This Before Posting


Topic: PH Calculation  (Read 12831 times)

0 Members and 1 Guest are viewing this topic.

chris p

  • Guest
PH Calculation
« on: July 18, 2005, 03:58:10 PM »
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.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:PH Calculation
« Reply #1 on: July 18, 2005, 04:39:41 PM »
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.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

chris p

  • Guest
Re:PH Calculation
« Reply #2 on: July 18, 2005, 04:46:38 PM »
Is there anyway to calculate the concentration (or Molarity) of the salt using the Ka?

Offline Donaldson Tan

  • Editor, New Asia Republic
  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3177
  • Mole Snacks: +261/-13
  • Gender: Male
    • New Asia Republic
Re:PH Calculation
« Reply #3 on: July 18, 2005, 04:56:05 PM »
we have to make assumption regarding the concentration of the salt.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline lemonoman

  • Atmospheric
  • Chemist
  • Full Member
  • *
  • Posts: 607
  • Mole Snacks: +71/-8
  • Gender: Male
Re:PH Calculation
« Reply #4 on: July 18, 2005, 06:09:51 PM »
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 :)

chris p

  • Guest
Re:PH Calculation
« Reply #5 on: July 18, 2005, 07:57:31 PM »
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.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:PH Calculation
« Reply #6 on: July 19, 2005, 04:44:21 AM »
Your answer is a way too high. Show your math.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

chris p

  • Guest
Re:PH Calculation
« Reply #7 on: July 19, 2005, 02:10:34 PM »
I tried again.

x^2/.5=Kb

-log OH = pOH

14-pOH = 8.5

Is this answer better?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:PH Calculation
« Reply #8 on: July 19, 2005, 03:18:35 PM »
Much better :)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

yl88

  • Guest
Re:PH Calculation
« Reply #9 on: August 13, 2005, 11:45:37 PM »
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...

Offline victor

  • Regular Member
  • ***
  • Posts: 84
  • Mole Snacks: +2/-0
  • Gender: Male
  • I'm a cat!
Re:PH Calculation
« Reply #10 on: August 15, 2005, 07:45:28 AM »
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

Offline sdekivit

  • Chemist
  • Full Member
  • *
  • Posts: 403
  • Mole Snacks: +32/-3
  • Gender: Male
  • B.Sc Biomedical Sciences, Utrecht University
Re:PH Calculation
« Reply #11 on: August 15, 2005, 11:36:32 AM »
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

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re:PH Calculation
« Reply #12 on: August 15, 2005, 11:59:13 AM »
a power can only be 1 when the power equals 0

0 = 1

Interesting result ;)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline sdekivit

  • Chemist
  • Full Member
  • *
  • Posts: 403
  • Mole Snacks: +32/-3
  • Gender: Male
  • B.Sc Biomedical Sciences, Utrecht University
Re:PH Calculation
« Reply #13 on: August 15, 2005, 12:03:51 PM »
0 = 1

Interesting result ;)

i mean x^0 is always 1 ;) sorry for the bad english. I meant the exponent ;)
« Last Edit: August 15, 2005, 12:04:45 PM by sdekivit »

Offline victor

  • Regular Member
  • ***
  • Posts: 84
  • Mole Snacks: +2/-0
  • Gender: Male
  • I'm a cat!
Re:PH Calculation
« Reply #14 on: August 16, 2005, 08:09:35 AM »
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

Sponsored Links