How Do You Calculate Formate Ion Concentration and pH in Mixed Solutions?

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Discussion Overview

The discussion revolves around calculating the formate ion concentration and pH of a mixed solution containing formic acid (HCOOH) and nitric acid (HNO3). Participants explore the implications of strong and weak acids in the context of equilibrium and dissociation in solutions.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asks how to set up the reaction equation for the interaction between formic acid and nitric acid.
  • Another participant suggests that knowing the hydronium ion concentration from nitric acid could be beneficial for calculations.
  • A participant mentions using the Ka expression for formic acid to find the concentration of hydronium ions, but their calculated pH does not match expected results.
  • It is noted that nitric acid is a strong acid that completely dissociates, providing a significant concentration of hydronium ions, which may influence the dissociation of formic acid.
  • Some participants express confusion about the correct approach to calculating pH and the dissociation of the acids involved.
  • One participant proposes that since formic acid is weak, it might be reasonable to ignore its contribution to the hydronium ion concentration and focus solely on the strong acid's concentration.
  • A later reply acknowledges this reasoning as a potential simplification that could save time in calculations.

Areas of Agreement / Disagreement

Participants exhibit uncertainty regarding the correct method for calculating pH in the presence of both strong and weak acids. Multiple viewpoints exist on how to approach the dissociation equations and the significance of each acid's contribution to the overall pH.

Contextual Notes

Some participants express confusion about the application of dissociation equations and the implications of strong versus weak acids on equilibrium. There is a lack of consensus on the correct approach to the calculations.

geffman1
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Homework Statement


calculate the formate ion concentration and pH of a solution that is 0.05m in formic acid (HCOOH;Ka=1.8x10^-4) and 0.10M in HNO3.


hey guys just wondering how the equation goes??

like this?? HCOOH + HNO3>>?
 
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Would knowing that nearly all of the hydronium ion concentration is from the nitric acid help you?
 
The equation you need is the equation for Ka. Try that one first...
 
when i tried doing the Ka i get x/(0.05*0.1)=1.8e-4 therefore x=3.6e-4 however when i -log3.6e-4 i get a ph of 3.44, however the answer is a pH of 1?? thanks for the replys
 
Last edited:
Nitric Acid is a strong acid; this means that its ions completely dissociate in water solution. You essentially have a solution which has 0.10 M in hydronium ion. What would be the pH of just that? How would 0.10 M hydronium affect the dissociation of the formic acid?
 
does that mean just make the dissocation equation just with HNO3. well this is what i did. HNO3>NO3 + H therefore equation equals. [H][NO3]/[HNO3]=1.8e-4 therefore solving this x^2/0.1=1.8e-4 therefore i get a ph of 2.37? thansk for replying
 
geffman1 said:
does that mean just make the dissocation equation just with HNO3. well this is what i did. HNO3>NO3 + H therefore equation equals. [H][NO3]/[HNO3]=1.8e-4 therefore solving this x^2/0.1=1.8e-4 therefore i get a ph of 2.37? thansk for replying

You are misapplying your information. Nitric acid is a strong acid. Formic acid is the weak acid. The concentration of nitric acid is high enough that it may push the dissociation of formic acid to the left in regard to HCHO <------> H+ + CHO-
 
i am confused haha, could u show me ur working for the dissocaition working please, if it is a weak acid isn't that's y the give the ka value for it. so does this just mean i used the dissociation expression for HCOOH and just used -log for HNO3? thansk
 
wow i think it just clicked, since HCOOH is weak, can we ignored it and just use the concentration of NO3 because its strong. because that works out. if that is right u just saved me so much time. thanksss heaps
 
  • #10
Fine! This indicates some good progress.
 

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