What is the percentage of nitrogen in urea (in $\%$)?

An organic compound was found to contain $40.0 \% C$ and $6.66 \% H$. Find its molecular formula (molar mass $= 180$).

In an analysis,the ratio of iodine to oxygen in a certain substance is $254:80$. What will be the formula of this substance? (Atomic mass $I = 127$,$O = 16$)

Calculate the molecular formula of an iron oxide,in which the mass percentages of iron and oxygen are $69.9$ and $30.1$,respectively.

One mole of one of the sodium salts listed below,having carbon content close to $14.3 \%$,produces $1 \ mole$ of carbon dioxide upon heating (atomic mass of $Na = 23, H = 1, C = 12, O = 16$). The salt is:

The value of $x$ in $KAl(SO_4)_x \cdot 12 H_2O$ is $.....$