Which of the following statements is not true about Mohr's salt?

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

Given below are two statements
Statement $I$: Aqueous solution of $K_2Cr_2O_7$ is preferred as a primary standard in volumetric analysis over $Na_2Cr_2O_7$ aqueous solution.
Statement $II$: $K_2Cr_2O_7$ has a higher solubility in water than $Na_2Cr_2O_7$.
In the light of the above statements,choose the correct answer from the options given below:

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

During the preparation of Mohr's salt solution (Ferrous ammonium sulphate),which of the following acid is added to prevent hydrolysis of $Fe^{2+}$ ion?

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