The $pH$ of pure water or neutral solution at $50\,^{\circ}C$ is .... ($pK_w = 13.26$ at $50\,^{\circ}C$)

Calculate the hydrogen ion concentration in the following biological fluids whose $pH$ are given below:
$(a)$ Human muscle-fluid,$6.83$
$(b)$ Human stomach fluid,$1.2$
$(c)$ Human blood,$7.38$
$(d)$ Human saliva,$6.4$

The concentration of hydrogen ion in pure water at $298 \ K$ is:

The $pH$ of a solution is the negative logarithm to the base $10$ of its hydrogen ion concentration in:

The $pH$ of a solution having $[OH^{-}] = 10^{-7} \ M$ is

The $pH$ of a solution is $9.95$. Calculate the $\left[OH^{-}\right]$ concentration.