Question

How to calculate the pH value of 0.0001 M `"HCl"` ?

Answer 1

`"pH" = 4.0`

Explanation:

Hydrochloric acid, `"HCl"`, is a strong acid, which means that it dissociates completely in aqueous solution to produce hydronium cations, `"H"_3"O"^(+)`, and chloride anions, `"Cl"^(-)`.

In order to calculate the pH of this solution, you need to know the concentration of hydronium cations.

The dissociation of the acid will produce

`"HCl"_ ((aq)) + "H"_ 2 "O" _ ((l)) -> "H"_ 3"O"_ ((aq)) + "Cl"_ ((aq))^(-)`

Here every mole of hydrochloric acid added to the solution will produce one mole of hydronium cations. In your case, you have

`["HCl"] = "0.0001 M" = 10^(-4)"M"`

This means that the concentration of hydronium cations is

`["H"_3"O"^(+)] = 10^(-4)"M"`

Plug this into the equation for pH

`color(purple)(bar(ul(|color(white)(a/a)color(black)("pH" = - log(["H"_3"O"^(+)]))color(white)(a/a)|)))`

to find the pH of the solution

`"pH" = - log(10^(-4)) = - (-4) * log(10) = 4.0`

The answer is rounded to one decimal place because you have one significant figure for the concentraiton of the solution.