What is the molecular mass of $NaOH$?

In an acidic aqueous solution,the value of $[H_3O^+]$ is $.......$.

What is meant by "water of crystallisation" in a substance? Explain with an example.

$A$ dry pellet of a common base $B$,when kept in open,absorbs moisture and turns sticky. The compound is also a by-product of the chlor-alkali process. Identify $B$. What type of reaction occurs when $B$ is treated with an acidic oxide? Write a balanced chemical equation for one such reaction.

Complete and balance the following chemical equations:
$(a)$ $Ca(OH)_{2} + Cl_{2} \longrightarrow$
$(b)$ $CaSO_{4} \cdot 1/2 H_{2}O + H_{2}O \longrightarrow$
$(c)$ $Na_{2}CO_{3} \cdot 10H_{2}O \xrightarrow{\text{air}} \dots$

$(a)$ Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity,illustrate how these two are interconvertible.
$(b)$ Write the chemical names and formulae of Plaster of Paris and gypsum.