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

1 Answer
Sep 5, 2016

Answer:

#"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.