Acid-Base Equilib: Solving Homework Problem

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Discussion Overview

The discussion revolves around a homework problem involving the calculation of pH and pOH for a sodium hydroxide solution prepared by dissolving 26g of NaOH in 150mL of water. Participants explore the correct method for calculating concentrations and the resulting pH and pOH values.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant calculates the number of moles of NaOH and questions whether they need to convert to mol/L.
  • Another participant corrects the initial misunderstanding about the dissociation of NaOH, clarifying that it produces OH- ions instead of H+ ions.
  • There is a discussion about the final concentration being slightly lower than initially calculated due to the increase in volume after dissolution, though some suggest this can be ignored.
  • Participants confirm the use of the formula c = n/v to find the concentration, emphasizing the importance of considering volume in calculations.

Areas of Agreement / Disagreement

Participants generally agree on the method of calculating concentration and the use of the relevant equations, but there is some confusion regarding the initial interpretation of the dissociation of NaOH and the impact of volume on concentration calculations.

Contextual Notes

Some participants note that the final concentration may be slightly lower than calculated due to volume changes, but this is not resolved. There is also an ongoing clarification about the correct ions produced by NaOH dissociation.

Who May Find This Useful

This discussion may be useful for students working on acid-base equilibrium problems, particularly those involving calculations of pH and pOH in solutions.

Veronica_Oles
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Homework Statement


To unclog a drain you add 26g of sodium hydroxide to 150mL of water. Calculate the pH and pOH for the solution you prepared. Can someone tell me if I'm doing it right.

Homework Equations

The Attempt at a Solution


n = m/M
n = 26/40
n = 0.65mol for 0.15L

Does this mean that I have to convert into mol/L?

So that would be 4.33 mol/L and this would also be my H+ value/ And then would I plug it into my equation

pH = -log[H+] and find my pH value.

Then plug my pH value into my pH + pOH = 14 equation and solve for pOH?

Is this the correct method?
 
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Veronica_Oles said:
So that would be 4.33 mol/L and this would also be my H+ value

No.

Trivial mistake, but you got it reversed. NaOH doesn't produce H+ when dissociating. Other than that, you are on the right track.

Actually the final concentration is a bit lower than 4.33 M as the volume is not 150 mL, it gets a bit higher after NaOH dissolution. But you can safely ignore it.
 
Borek said:
No.

Trivial mistake, but you got it reversed. NaOH doesn't produce H+ when dissociating. Other than that, you are on the right track.

Actually the final concentration is a bit lower than 4.33 M as the volume is not 150 mL, it gets a bit higher after NaOH dissolution. But you can safely ignore it.
Sorry I meant to put OH-

okay here's what I've gotten

NaOH = 26/40 = 0.65mol

And 0.65 mol would be my 0.65 mol/L?
 
Veronica_Oles said:
Sorry I meant to put OH-

okay here's what I've gotten

NaOH = 26/40 = 0.65mol

And 0.65 mol would be my 0.65 mol/L?

I put that value into my pOH equation and ended up with 0.187

I then put that into the next equation
pH = 14 - pOH and got
pH = 13.81

Is this correct or way off?
 
Veronica_Oles said:
NaOH = 26/40 = 0.65mol

And 0.65 mol would be my 0.65 mol/L?

No, you have to take the volume into account, Actually you did it - and correctly - in your first post.
 
Borek said:
No, you have to take the volume into account, Actually you did it - and correctly - in your first post.
Okay so basically I'm using the formula c= n/v?
 
Yes, just like you did initially.
 

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