Which of the following cannot give iodometric titrations?

The ammonia evolved from the treatment of $0.30 \, g$ of an organic compound for the estimation of nitrogen was passed in $100 \, mL$ of $0.1 \, M$ sulphuric acid. The excess of acid required $20 \, mL$ of $0.5 \, M$ sodium hydroxide solution for complete neutralization. The organic compound is:

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

Assertion : $K_2Cr_2O_7$ is used as a primary standard in volumetric analysis.
Reason : It has a good solubility in water.

$20 \, mL$ of a solution of a weak monobasic acid neutralizes $22.18 \, mL$ of a solution of $NaOH$,and $20 \, mL$ of $N/10$ $HCl$ neutralizes $21.5 \, mL$ of the same $NaOH$ solution. The normality of the acid is nearly $.......$ $N$.

$K_2Cr_2O_7$ is preferred to $Na_2Cr_2O_7$ for use in volumetric analysis as a primary standard because