Investigating the Reaction of HCl with Water

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Discussion Overview

The discussion revolves around the dissolution of hydrogen chloride (HCl) in water, focusing on the resulting chemical reaction, the calculation of pH, and the concepts of molar concentration in the context of a homework problem.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant presents the reaction equation for HCl dissolving in water as HCl + H2O ------> H3O+ + Cl-.
  • Another participant suggests applying the pH definition to find the pH of the solution.
  • There is uncertainty expressed regarding the molar concentration of H3O+, with participants discussing the need for known values to calculate it.
  • Participants clarify that molar concentration is defined as the number of moles divided by volume, but there is confusion about the specific values provided in the problem.
  • One participant calculates the molar concentration of H3O+ as 0.05 mol/L and subsequently attempts to calculate the pH as -log(0.05) = 1.3.
  • There is a question about whether the molar concentration of Cl- is the same as that of H3O+, which is affirmed based on the stoichiometry of the dissociation reaction.

Areas of Agreement / Disagreement

Participants express confusion and uncertainty regarding the calculations and the definitions involved. There is no consensus on the clarity of the problem or the calculations, as some participants are lost in the discussion of moles and liters.

Contextual Notes

Participants highlight limitations in the information provided, particularly regarding the relationship between the moles of HCl and the volume of water, which complicates the calculation of molar concentrations.

Who May Find This Useful

This discussion may be useful for students learning about acid-base chemistry, molar concentration, and pH calculations, particularly in the context of homework problems involving solutions.

chawki
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Homework Statement


When the gas hydrogen chloride (HCl) dissolves in water it acts as a strong acid.

Homework Equations


a) Write the reaction equation for hydrogen chloride dissolving in water.
b) Find the pH of the solution formed when 0.040 mol of hydrogen chloride gas dissolves in
0.80 litres of pure water at a temperature of 298.15 K.

The Attempt at a Solution


a) HCl + H2O ------> H3O+ + Cl-
 
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Now apply pH definition.
 
yes but we don't know the molar concentration of H3O+
 
Ah, so you have been sleeping through the concentration classes. What is the definition of a molar concentration?
 
number of moles/volume.
and we don't know any of them
 
chawki said:
number of moles/volume.
and we don't know any of them

What can you measure volume in? (units)
 
chawki said:
0.040 mol of hydrogen chloride gas dissolves in 0.80 litres of pure water

chawki said:
number of moles/volume.
and we don't know any of them

Which one you are not given in the question?
 
Litres of course.
 
Borek said:
Which one you are not given in the question?

i don't get it and I'm lost when they give moles with litres
 
  • #10
Molar concentration is number of moles in 1 liter, to calculate concentration you just divide number of moles (expressed in moles) by volume (expressed in liters). It can't be simpler.
 
  • #11
If it was the moles and the litres of the same compound..that would be easy..but we have the moles of HCl and the litres of H2O
 
  • #12
Are you saying that the Molar concentration of H3O+= 0.040/0.8 ?
and that would be 0.05mol/l

and then, PH=-log0.05 =1.3 ?
 
  • #13
chawki said:
Are you saying that the Molar concentration of H3O+= 0.040/0.8 ?
and that would be 0.05mol/l

Right. Told you you were sleeping through concentration classes. While it is possible to calculate concentration of substance in the same substance it is rarely needed and rarely done. In most cases we are interested in concentration of something in a mixture. In this case - concentration of HCl in the solution.

and then, PH=-log0.05 =1.3 ?

OK
 
  • #14
Thank you so much Borek
is the Molar concentration of Cl- same as H3O+
 
  • #15
chawki said:
is the Molar concentration of Cl- same as H3O+

In this case - yes. That's a simple effect of dissociation stoichiometry - there is one Cl- produced per each H3O+.
 

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