Chemistry NCERT Exemplar Solutions Class 12th Chapter One

4. Let the formula of sample be (Fe²⁺)x (Fe³⁺) yO

From the formula of the compound the equation can be formed as-

x+ y= 0.93 . (i) 

Total positive charge on ferrous and ferric ions should balance the two units of negative charge on oxygen. Hence,  

2x+ 3y= 2 . (ii) 

⇒ x+ $\frac{3}{2}$  y=1. (iii)

On subtracting equation (i) from equation (iii) we have

$\frac{3}{2}$ y-y=1 - 0.93

12y=0.07

y= 0.14 

On putting the value of y in equation (i)

 we get, x+ 0.14 = 0.93 

⇒x = 0.93 – 0.14 

⇒x = 0.79 

Fraction of Fe²⁺ ions present in the sample is  $\frac{0.79}{0.93}$  = 0.81

Metal deficiency defect is found to be present in the sample as iron is less in amount than that required for stoichiometric composition.

3. The conductivity of a semiconductor is very low and it can be increased by adding small impurities and this process is known as doping. 

Doping can be done with an impurity which is electron rich or electron deficient-

n-type semiconductors- Si or Ge (group-14 elements)  are doped with electron rich impurity (group-15 elements like P or As)  and are known as n-type semiconductors. In them the conductivity is due to the extra electron or delocalized electron. 

When intrinsic semiconductors like Si or Ge are doped with pentavalent elements like P or As, they occupy some of the lattice sites in silicon or germanium crystal . Four out of five electrons are used in formation of four covalent bonds with four neighbouring silicon atoms. The fifth electron is extra and gets delocalised. These delocalised electrons increase the conductivity of doped silicon ( or germanium).

p-type semiconductors- Si or Ge (group-14 elements) doped with electron deficient impurity (group 13 elements like B or Al or Ga) and are termed as  p-type semiconductors. Here the conductivity is due to the positively charged electron holes. 

In this case the doping of intrinsic semiconductors like silicon or germanium with trivalent elements like B / In / Ga, three out of four electrons in silicon or germanium form bonds with doping impurity ( i.e. B/ In / Ga ). The fourth electron remains unbonded. The place where the fourth valence electron is missing is known as electron hole or electron vacancy.

An electron from a neighbouring atom can come and fill up the electron hole, but in doing so it would leave an electron hole at its original position. If this happens it would appear that the electron hole has moved in a direction opposite to that of the electron that filled it. Under the influence of electric field electrons would move towards the positively charged plate through electron holes, but it would appear as if electrons were positively charged and are moving towards the negatively charged plate.