1. Why chemical reactions are in …

(1) In any chemical reaction, the total mass of the reactants is equal to the total mass of the products. In a chemical reaction, only rearrangement of atoms takes place which means that mass can neither be created nor be destroyed. Thus, chemical reactions are in accordance with law of conservation of mass.  

(2) 10 g of Ne gas = 6.022 x 10²³ atoms as Neon is a monoatomic gas.

(3) 1 mol of H₂ = 1g H₂ = 2 x 6.022 x 10²³ atoms = 12.044 x 10²³atoms

1 mol of He = 4g He = 4 x 6.022 x 10²³ atoms = 2 x 12.044 x 10²³ atoms

This is because Hydrogen is a diatomic molecule whereas Helium is monoatomic.

Ans. 

1. Chemical reactions follow the law of conservation of mass because the reactants and products are chemical substances which are basically nothing but matter, and by the law of conservation of mass matter can neither be created nor be destroyed. Therefore chemical reaction follow the law of conservation of mass.
2. Atomic mass in gram = 1 mole = 6.023 * 10²³.
   Since Neon is Mono atomic in that case gram atomic mass will be equal to 1 mole.
   Atomic Mass of Ne = 20 gram = 1 mole = 6.023 * 10²³