State the postulate of Dalton's atomic theory which led to the law of conservation of mass. Calculate the molecular mass of water.

$(a)$ $A$ sample of vitamin $C$ is known to contain $2.58 \times 10^{24}$ oxygen atoms. How many moles of oxygen atoms are present in the sample?
$(b)$ Write one word for the following:
$(i)$ In a balanced chemical equation,the sum of the masses of reactants and products remains unchanged.
$(ii)$ $A$ group of atoms carrying a fixed charge on them.
$(c)$ Write chemical formulae of the following compounds:
$(i)$ Sodium phosphate
$(ii)$ Ammonium carbonate

$(a)$ In this crossword puzzle (Fig),names of $11$ elements are hidden. Symbols of these are given below. Complete the puzzle.
$1. Cl$
$2. H$
$3. Ar$
$4. O$
$5. Xe$
$6. N$
$7. He$
$8. F$
$9. Kr$
$10. Rn$
$11. Ne$
$(b)$ Identify the total number of inert gases,their names,and symbols from this crossword puzzle.

$3.42 \, g$ of sucrose are dissolved in $18 \, g$ of water in a beaker. The number of oxygen atoms in the solution are

Give three differences between an atom and a molecule.

$A$ $0.24 \, g$ sample of a compound of oxygen and boron was found by analysis to contain $0.096 \, g$ of boron and $0.144 \, g$ of oxygen. Calculate the percentage composition of the compound by weight.