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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: robotnel on April 24, 2014, 10:51:19 PM

Title: Deriving a pH value from molarity
Post by: robotnel on April 24, 2014, 10:51:19 PM

I'm working on a lab report tonight and I can't seem to figure out if my maths is right. Every time I rework the problem, I get a different answer.

Here are the facts:

• 25 mL of 0.10 M NH₃ is mixed with 25 mL of 0.10 M NH₄NO₃
• 10 mL of the resulting solution was tested for pH with a Vernier
• Observed pH was 9.87
• NH3 Kb = 1.77x10^-5

I'm having quite a difficulty arriving at a calculated pH. I think the volume is throwing me off (maybe it's a red herring?). I also need to the [ H+ ] concentration.

Durn ice charts and Kb values!

Any help or pointer in the right direction would be appreciated. I believe I have calculated my [HA] concentration incorrectly when I try to do my Henderson-Hasselbach equation.

Thank you.

p.s. The calculated pH I have is 10.97

Title: Re: Deriving a pH value from molarity
Post by: Borek on April 25, 2014, 02:44:28 AM

Hard to say what you did wrong, not seeing what you did.

Observed pH looks reasonably accurate to me, calculated one is off by about 1 pH unit.