When a standard solution of $NaOH$ is left in the air for a few hours,

Consider the following statements:
$(a)$ The $pH$ of a mixture containing $400 \, mL$ of $0.1 \, M \, H_2SO_4$ and $400 \, mL$ of $0.1 \, M \, NaOH$ will be approximately $1.3$.
$(b)$ Ionic product of water is temperature dependent.
$(c)$ $A$ monobasic acid with $K_a = 10^{-5}$ has a $pH = 5$. The degree of dissociation of this acid is $50 \%$.
$(d)$ The Le Chatelier's principle is not applicable to common-ion effect.
The correct statements are:

In which of the following acid-base titrations is the $pH$ at the equivalence point greater than $8$?

The hydrogen ion concentration in a mixture of $10 \ mL$ of $0.1 \ M$ $H_2SO_4$ and $10 \ mL$ of $0.1 \ M$ $KOH$ solution in water,is . . . . . . $M$.

Estimate the approximate $pK_a$ of $0.5 \ M$ $CH_3COOH$. Degree of dissociation (ionisation) is $0.15$ $(\log 1.32 = 0.12)$.

The $pH$ of a solution obtained by mixing $50 \, mL$ of $1 \, M \, HCl$ and $30 \, mL$ of $1 \, M \, NaOH$ is $x \times 10^{-4}$. The value of $x$ is ...... . (Nearest integer) $[\log 2.5 = 0.3979]$