<h2>Answer : Law of conservation of mass</h2><h3>Explanation :</h3>
The law of conservation of mass states that in any reaction mass is neither created nor lost it has to remain constant in a system.
In this case, when the reaction setup was done in normal way the mass was lost in surrounding was not considered nor being calculated; whereas when the reaction was studied in a closed system where the gas was collected after the reaction the mass changes was noted down which helped to prove the point of law of conservation of mass and energy.
One can consider an example of soda can where the carbonated drink contains pressurized carbon dioxide gas. when opened the gas bubbles gets lost into the surroundings and we don't measure the mass changes. Instead if the soda can was opened in such a way where the gas evolved was measured then the mass changed would remain the same.
Answer:
Oxidation number of F2O = 0−(−1×2)
State of oxygen will be=+2
Explanation:
As 
is a covalent compound because it is made up by the combination of two non-metal atoms. Atomic number of an iodine atom is 53 and it contains 7 valence electrons as it belongs to group 17 of the periodic table.
Therefore, sharing of electrons will take place when two iodine atoms chemically combine with each other leading to the formation of a covalent bonding.
Hence, weak forces like london dispersion forces will be present between a molecule of .
The weak intermolecular forces which can arise either between nucleus and electrons or between electron-electron are known as dispersion forces. These forces are also known as London dispersion forces and these are temporary in nature.
thus, we can conclude that london dispersion force is the major attractive force that exists among different molecules in the solid.
