Read the following passage and answer the questions at the end of it. Conductivity of silicon increases if it is doped with certain other elements. Doping means introduction of small amount of impurities like phosphorus, arsenic or boron in the pure crystal. In pure silicon four valencies are used in bonding with other four adjacent silicon atoms. When a silicon crystal is doped with a group - 15 element (with five valence electrons) such as P, AsSb or Bi, the structure of the crystal lattice remains unchanged. Out of the five valence electrons of group - 15 doped element, four electrons are used in normal covalent bonding with silicon and fifth electron is delocalised and thus conducts electricity
Doping a silicon crystal with a group-13 element (with three valence electron) such as B, Al, Ga or In produces a semiconductor with three electrons in dopant. The place where fourth electron is missing is called as electron vacancy or hole.
If NaCl is doped with `10^(-3)" mol % " SrCl_(2)` then concentration of cation vacancies is 

Options

A. `6.02xx10^(23)`
B. `6.02xx10^(20)`
C. `6.02xx10^(18)`
D. `6.02xx10^(15)`

Answer

C

Explanation

For each `Sr^(+2)` one cation vacancy is created. Doping of `NaCl` with `10^(-3)` mol `% SnCl_(2)` means that 100 moles of `NaCl` are doped with `10^(-3)` moles of `SrCl_(2)`
`"100 mole NaCl"rarr10^(-3)" mole of "SrCl_(2),"1 mole "NaClrarr?(10^(-3))/(100)=10^(-5)" moles"`
`=10^(-5)` mole/mole of `NaCl=10^(-5)xx6xx10^(13)=6xx10^(18)" mol"^(-1)`.