Write the names of the following compounds:
$(a)$ $NiS$
$(b)$ $Mg(NO_3)_2$
$(c)$ $Na_2SO_4$
$(d)$ $Al(NO_3)_3$
$(e)$ $K_3PO_4$
$(f)$ $Ca_3N_2$

$(a)$ What is the law of constant proportions? Explain with the help of an example.
$(b)$ Which postulate of Dalton's atomic theory is in agreement with this law?

$A$ 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?

The symbols of calcium,carbon,and copper are

Fill in the blanks:
$(a)$ In a chemical reaction,the sum of the masses of the reactants and products remains unchanged. This is called ...............
$(b)$ $A$ group of atoms carrying a fixed charge on them is called ...............
$(c)$ The formula unit mass of $Ca_3(PO_4)_2$ is ...............
$(d)$ The formula of sodium carbonate is ............... and that of ammonium sulphate is ...............

Which of the following postulates of Dalton's atomic theory is challenged by the next generation of scientists?