What is the composition of the white enamel of our teeth?

If $10^{21}$ molecules are removed from $100 \, mg$ of $CO_2$,how many moles of $CO_2$ remain?

When iron sulfide is heated in air,a sulfur oxide $A$ is obtained. When $A$ is dissolved in water,it produces an acid. What is the basicity of this acid?

The incorrect postulates of the Dalton's atomic theory are :
$A$. Atoms of different elements differ in mass.
$B$. Matter consists of divisible atoms.
$C$. Compounds are formed when atoms of different element combine in a fixed ratio.
$D$. All the atoms of given element have different properties including mass.
$E$. Chemical reactions involve reorganisation of atoms.
Choose the correct answer from the options given below :

If $1 \ g$ of each of the following gases are taken at $STP$,which of the gases will occupy $(a)$ greatest volume and $(b)$ smallest volume? $CO, H_2O, CH_4, NO$

Which of the following species has the highest mass?