The mass percentage of nitrogen in histamine is ___________.

We start by recalling the molecular formula of histamine. Histamine is known to be $$\mathrm{C_5H_9N_3}$$, that is, each molecule contains 5 carbon atoms, 9 hydrogen atoms and 3 nitrogen atoms.

Now, to find the mass percentage of nitrogen, we first need the molar mass of histamine. We list the atomic masses (in g mol−1) that will be used:

$$\text{Atomic mass of C}=12.01,\qquad\text{Atomic mass of H}=1.008,\qquad\text{Atomic mass of N}=14.01$$

We next calculate the total mass contributed by each element in one mole of histamine.

For carbon we have 5 atoms, so

$$m_{\text{C}} = 5 \times 12.01 = 60.05$$

For hydrogen we have 9 atoms, so

$$m_{\text{H}} = 9 \times 1.008 = 9.072$$

For nitrogen we have 3 atoms, so

$$m_{\text{N}} = 3 \times 14.01 = 42.03$$

The molar mass of histamine is obtained by adding these three contributions:

$$M_{\mathrm{C_5H_9N_3}} = 60.05 + 9.072 + 42.03 = 111.152\;\text{g mol}^{-1}$$

We now apply the general formula for percentage composition of an element in a compound:

$$\text{Mass percentage of an element} = \frac{\text{Total mass of that element in one mole of compound}}{\text{Molar mass of the compound}}\times 100$$

Substituting the values for nitrogen we get

$$\%\ \text{N} = \frac{42.03}{111.152}\times 100$$

Carrying out the division first,

$$\frac{42.03}{111.152} \approx 0.378$$

and then multiplying by 100,

$$0.378 \times 100 = 37.8$$

So, the answer is $$37.8\%$$.