Calculate the pH of a 6.2 x 10-11 M HCl solution

Question

 a) - 0.60

 

  b) 13.30

 

Correct!

  c) 7.00

 

  d) 0.99

 

Answer 1

pH = -log [H+]

[H+] = [H+] from H2O + [H+] from HCl

[H+] from H2O (@25ºC) = 1x10-7 M

[H+] from HCl = 6.2x10-11 M

∑ = 1x10-7 + 6.2x10-11 = 1.00062x10-7

pH = -log 1.00062x10-7

pH = 6.9997 = 7.00 (2 sig. figs.) ANSWER (C)

Answer 2

Hello, Alessio,

[NOTE: My explanation is incorrect. Please use the method outlined by J.R. S. - Sorry]

The definition of pH is "the negative log of the hydrogen ion concentration."

This can be written as −log₁₀ c, where c is the hydrogen ion concentration in moles per liter.

So we need to find the concentration of H⁺ in 6.2 x 10^-11M HCl. HCl is a strong acid, so it will completely dissolve in water, especially with such a low concentration.

HCl = H⁺ + Cl^-

This tells us that we get equal numbers of H⁺ and Cl^- atoms, and that number is the same as the number of HCL molecules. So if we start with 6.2 x 10^-11M HCl, we'll have 6.2 x 10^-11M moles each of the two ions.

Therefore [H⁺] = 6.2 x 10^-11M

The log of 6.2 x 10^-11M is -10.2

The negative log is therefore 10.2, and that is the pH.

Bob [See response by J.R.S. for the correct calculation.