6A.21

[Kiiana Hilario 2E]

Why do we assume that [H3O+] and [OH-] are the same value in part A?

[Erin Haisan 1B]

Hello! We assume that [H3O+] and [OH-] are the same value since we are looking for each concentration in a neutral sample of water, meaning that [H3O+]=[OH-] (neutral solution, since there is not a greater value of H3O or OH). I hope this helps!

[Stanley Munoz 3F]

Hi! In this example, we are given the Kw, a neutral sample of water. Also, remember that Ka x Kb=Kw which can also be formatted as [H30+][OH-]=1x10^-14; therefore, the concentrations of both hydronium and hydroxide must have the same concentrations when it comes to the neutral sample of water. Hope this helps!

[Anna_Aldridge_2C]

Like others have said, the concentration of hydronium [H3O+] and hydroxide [OH-] ions will be equal for neutral water. A helpful way to remember the concentrations for neutral water is to simply remember the pH of neutral water, 7. 7 is in the center of the pH scale of 0 to 14, meaning the concentrations will be equal because the [H3O+] times [OH-] (10^-7 times 10^-7) will equal 1 x 10^-14, or a pH of 14. The question asks for concentration of hydroxide, so the answer would be 10^7.

Hope this helps!