Identify the number of molecules in which the central atom has one lone pair of electrons from the following list.

$$ \mathrm{PbCl}_2, \mathrm{PH}_3, \mathrm{ClF}_3, \mathrm{SF}_4, \mathrm{BF}_3, \mathrm{SnCl}_2 $$

In which of the following molecules, the number of lone pairs of electrons on central atom and the number of $d$-orbitals involved in the hybridisation of central atom, is same?

4 g of an ideal gas $A$ (molar mass $=M_A$ ) present in a vessel of volume $V$ litre exerted a pressure of 5 atm at 300 K . When 16 g of another ideal gas $B$(molar mass $=M_B$ ) was introduced into this vessel at the same temperature, its pressure increased to 10 atm . What is the correct relationship between $M_A$ and $M_B$ ?

The sum of three values $12.0,19.034$ and 2.0143 is equal to $X$. The number of significant figures in $X$ is