This thermochemical equation needs to be balanced. Hence, option B is correct.
<h3>What is a balanced chemical equation?</h3>
A balanced equation contains the same number of each type of atom on both the left and right sides of the reaction arrow.
The balanced thermochemical equation is:
→ 
Hence, option B is correct.
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Answer: hydrogen
Explanation: hydrogen gas is a major component of water which occupies a large portion of the Earth's atmosphere
Mass of molecule (g) = Mr of substance over avarogado constant
Answer:K2X
Explanation: Valency can be defined as the combining power of an element. It is the valency that dictates the value an element will have when writing a chemical formula for its compound.
MgX is a compound of magnesium and an element X. The valency of magnesium in most of its compound is +2. Now for the 2 to have been absent in the chemical formula, this shows that the element X itself have a valency if -2 for the valencies of both to have canceled out.
Now considering the element potassium, it is an alkaline metal belonging to group 1 of the periodic table. Hence, it is expected that it has a valency of +1
Forming a compound with element X means there would be an exchange of valencies between the two. We have established that x has a valency of -2. The formula of the compound thus formed by exchanging the valencies of both element would be K2X