at a particular temperature, asample of pure water has a Kw of 1.7x10-12. what is the hydroxide concentration of this sample

Hydroxide concentration of the sample is 1.3x10⁻⁶M

Explanation:

The equilibrium constant of water, Kw, is:

H₂O(l) ⇄ H⁺(aq) + OH⁻(aq)

Kw is defined as:

Kw = 1.7x10⁻¹² = [H⁺] [OH⁻]

As the sample is of pure water, both H⁺ and OH⁻ ions have the same concentration because come from the same equilibrium, that is:

[H⁺] = [OH⁻]

We can write the Kw expression:

1.7x10⁻¹² = [OH⁻] [OH⁻]

1.7x10⁻¹² = [OH⁻]²

1.3x10⁻⁶M = [OH⁻]

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)