16. The valency of sodium is +1 and that of chlorine is -1, why?​

Explanation:

Since sodium has only one electron in the outermost shell which takes part in the bonding its valency is +1 . Chlorine atoms can accept the electron in 3p orbital to attain the nearest noble gas configuration of Ar . Chlorine has 7 valence electrons. It requires one more electron to complete its octet.

option c: 1605.1 kj

the balanced chemical reaction for combustion of methane is as follows:

here, enthalpy formation of is -74.7 kj/mol, 0 kj/mol, -393.5 kj/mol and -241.82 kj/mol respectively.

the heat released or can be calculated as follow:

putting the values,

thus, heat released by combustion of 1 mol of methane is 802.54 kj/mol. thus, for 2 mol, heat released will be:

therefore, heat released by the combustion of 2 mol of methane is 1605.1 kj.