Calculation steps:

• Number of atoms in $0.8\text{ mol}$ of substance:
  $$N = 0.8 \times N_A = 0.8 \times 6.022 \times 10^{23} = 4.8176 \times 10^{23}\text{ atoms}$$
• In any close-packed lattice (hcp or ccp), each constituent particle contributes 2 tetrahedral voids:
  $$\text{Number of tetrahedral voids} = 2 \times N$$
  $$\text{Number of tetrahedral voids} = 2 \times 4.8176 \times 10^{23} = 9.6352 \times 10^{23} \approx 9.635 \times 10^{23}$$

Defect characteristics of Potassium Chloride ($\text{KCl}$):

• Schottky Defect (A): $\text{KCl}$ is an ionic solid with high coordination number and cations ($\text{K}^+$) and anions ($\text{Cl}^-$) of comparable size, making it show Schottky defect (equal numbers of cation and anion vacancies).
• Metal Excess Defect (C): When $\text{KCl}$ crystals are heated in potassium vapor, excess potassium atoms deposit on the crystal surface and $\text{Cl}^-$ diffuses out, leaving an electron trapped in the anion vacancy ($F$-centre), which imparts a violet/lilac color. Thus, $\text{KCl}$ exhibits metal excess defect.
• $\text{KCl}$ does not show Frenkel defect (due to small size difference between ions) or metal deficiency defect (typical of transition metal oxides like $\text{FeO}$).