How to Calculate PH and POH when Adding Solid KOH to HCL Solution?

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Discussion Overview

The discussion revolves around calculating the pH and pOH of a solution formed by adding solid KOH to an HCl solution. Participants explore the necessary calculations involving molarity, mass-to-moles conversion, and the implications of excess reactants in determining the acidity or alkalinity of the resulting solution.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about converting grams of KOH to moles and finding concentration, noting the relationship between pH and concentration.
  • Another participant confirms the mass-to-moles calculation for KOH but emphasizes the need to find the excess moles of KOH or HCl to determine if the solution is acidic or alkaline.
  • There is a correction regarding the calculation of moles from mass, with a participant clarifying that the correct method involves dividing the mass of KOH by its molar mass.
  • A participant calculates that 0.2g of KOH corresponds to 0.003 moles and compares it to the moles of HCl present, suggesting that the pH would be 7 if they neutralize each other completely.
  • Concerns are raised about what to do if there is an excess of one reagent, with questions about calculating the volume of KOH without knowing its density.
  • Another participant suggests that density is not necessary for this calculation since the volume change from adding a small amount of solid is negligible.

Areas of Agreement / Disagreement

Participants generally agree on the method for converting mass to moles and the implications of neutralization. However, there is some disagreement regarding the necessity of density in calculating the volume of KOH and the implications of excess reagents.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the volume change when adding solid KOH and the need for density in calculations, which remain unresolved.

vileofmsz
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1. Calculate the PH and the POH of the solution obtained by adding .2g of Solid KOH to 1.5 liters of .002M HCL

Homework Equations


3.I understand this is a mol/ liter question. but I just don't understand how to transfer .2g of koh to ml and to find the concentration. I know PH= - log [Concentration Someone please help

I got KOH= 56.1g/mol. so one gram of KOH must = to 56.1 moles. If i multiply by .2 I get that .2g of KOH =11.22g
 
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Your mass to moles for KOH is correct. Next, find the excess number of moles of either KOH or HCl, and this will tell you if the solution is acidic (for which you find pH), or alkaline (for which you find pOH). To find the corresponding other pX value, you use pH+pOH=14
 
vileofmsz said:
I got KOH= 56.1g/mol. so one gram of KOH must = to 56.1 moles. If i multiply by .2 I get that .2g of KOH =11.22g

No, that's not OK. Why do you multiply mass of the KOH by the molar mass of the KOH? 11.22 is not number of moles, besides, it is not in g - it is in g2mol-1.

Check this calculation of pH of mixture example.
 
My mistake is the first sentence:
symbolipoint said:
Your mass to moles for KOH is correct. Next, find the excess number of moles of either KOH or HCl, and this will tell you if the solution is acidic (for which you find pH), or alkaline (for which you find pOH). To find the corresponding other pX value, you use pH+pOH=14

Correct would be (0.2 grams KOH)/(56.1 grams KOH / mole KOH)= theMolesOf KOH.
Read that text carefully; that is supposed to be a division of 0.2 by 56.1
Note the slash mark (actually two of them. The first is for the numbers, the second is for a ratio)
 
So 0.2g KOH/56.1g/mol KOH = .003 mol and 0.002M HCL * 1.5L HCL = 0.003 mol.

So 0.003 mol KOH - 0.003 mol HCl =0, so the pH has to be 7, right?

But what if there was still some of a reagent left? Then you would need to calculate the volume of KOH, correct? And how would you do that when you don't have the density (D=mV)?
 
webz said:
So 0.2g KOH/56.1g/mol KOH = .003 mol

Don't round down intermediate results.

But what if there was still some of a reagent left? Then you would need to calculate the volume of KOH, correct? And how would you do that when you don't have the density (D=mV)?

You don't need density here - you know there is 1.5L of solution. Volume almost doesn't change when adding so small amount of solid reactant.
 

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