Why has monochloroethanoic acid higher `pK_(a)` value than dichloroethanoic acid ?

State reasons for the following situations : Monochloroethanoic acid is a weaker acid than dichloroethanoic acid.

Chloroacetic acid has lower pK_(a) value than acetic acid.

Trimethylacetic acid has ............. pk_a value than trichloroacetic acid.

o-Nitrophenol has lower/higher pk_(a) value than m-nitrophenol.