Why is it essential to wash the precipitate with water before estimating it quantitatively?

$30 \ mL$ of acid solution is neutralized by $15 \ mL$ of a $0.2 \ N$ base. The strength of acid solution is $.............. \ N$.

Why is it necessary to wash the precipitate before measuring it quantitatively?

When potassium permanganate is titrated against ferrous ammonium sulphate,the equivalent weight of potassium permanganate is

$500 \ mL$ of $1.2 \ M$ $KI$ solution is mixed with $500 \ mL$ of $0.2 \ M$ $KMnO_4$ solution in basic medium. The liberated iodine was titrated with standard $0.1 \ M$ $Na_2S_2O_3$ solution in the presence of starch indicator till the blue color disappeared. The volume (in $L$) of $Na_2S_2O_3$ consumed is . . . . . . . (Nearest integer)

The weight of oxalic acid that will be required to prepare a $1000 \ mL$ $(N / 20)$ solution is