Investigating the Reaction of HCl with Water

[chawki]

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^-

 

[Borek]

Now apply pH definition.

 

[chawki]

yes but we don't know the molar concentration of H₃O⁺

 

[Borek]

Ah, so you have been sleeping through the concentration classes. What is the definition of a molar concentration?

 

[chawki]

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

 

[sjb-2812]

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

What can you measure volume in? (units)

 

[Borek]

0.040 mol of hydrogen chloride gas dissolves in 0.80 litres of pure water

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

Which one you are not given in the question?

 

[chawki]

Litres of course.

 

[chawki]

Which one you are not given in the question?

i don't get it and I'm lost when they give moles with litres

 

[Borek]

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.

 

[chawki]

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

 

[chawki]

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 ?

 

[Borek]

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

 

[chawki]

Thank you so much Borek
is the Molar concentration of Cl^- same as H₃O⁺

 

[Borek]

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⁺.