At 40°c, the ion-product constant of water, kw, is 2.88×10-14. what is the ph of pure water at 40°c?

Answer is: pH value of pure water at 40°C is 6.77.
Kw = 2.88·10⁻¹⁴.
Kw = [H⁺] · [OH⁻]. 
[H⁺] = [OH⁻] = x.
Kw = x².
x = [H⁺] = √2.88·10⁻¹⁴.
[H⁺] = 1.7·10⁻⁷ M.
pH = -log[H⁺].
pH = -log(1.7·10⁻⁷ M) = 6.77.

Compound with sulfer  is bright red