Question

How can you find the pH if 75 grams of HCl (hydrochloric acid) is put in 5.0L of water?

I understand that I have to convert the grams to mol, and I have the molarity, but I don't know if the concentration is of hydrogen or hydroxide ions. Can anybody help me? Thanks!

Answer 1

Well, let's see...I gets `pH=0.386`

Explanation:

We assume that hydrogen chloride undergoes complete protonolysis in aqueous solution...

`HCl(g) +H_2O(l) rarrH_3O^+ + Cl^-`

And so we calculate the concentration of `HCl` in the usual way...

`[HCl]="Moles of HCl"/"Volume of solution"`

We ASSUME (reasonably) that the volume of THE SOLUTION is `5.0*L`...it would be very close...

And so...`[HCl]=((75*g)/(36.46*g*mol^-1))/(5.0*L)=0.411*mol*L^-1`

And so it is `0.411*mol*L^-1` with respect to hydronium ions... which we represent as `H_3O^+` or `H^+`...

`pH=-log_10[H_3O^+]=-log_10(0.411)=-(-0.386)=+0.386`

The maximum concentration of `HCl` is approx. `10.6*mol*L^-1`...what is `pH` here...?