Fill in the blanks:
$(1)$ In the crystal structure of ice, the oxygen atom is at a distance of $276 \ pm$ from four hydrogen atoms.
$(2)$ The relative proportion of tritium to protium is $1: 10^{18}$.

How can the production of dihydrogen,obtained from 'coal gasification',be increased?

The method for preparation of water gas is

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Isotopes of hydrogen have almost same chemical properties,but difference in their rates of reaction.
Reason $R:$ Isotopes of hydrogen have different enthalpy of bond dissociation.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following substances is used as a dehydrating agent in the laboratory?

Consider the following sequence of reactions: $Na + NH_{3(g)} \to [X]$ $\xrightarrow{N_2O} [Y]$ $\xrightarrow{\text{Heat}} [Z] (\text{Gas, Pure})$. Identify $[Z]$ gas.