Percent Composition By Mass

Using only the molar mass of nitrogen, $14.007\,g/mol$, divided by the molar mass of calcium, which yields an irrelevant ratio.

Multiplying the nitrogen atomic mass by two and then adding the mass of oxygen, without proper division by total mass, misrepresenting the percent composition.

Adding the atomic masses of calcium and nitrogen without considering the nitrate group, resulting in an erroneous percentage.

Calculating total molar mass as $40.078 + 2*(14.007+ 3*16) = 164.092\,g/mol$ and then computing percent nitrogen as $\frac{2*14.007}{164.092} \times 100 \approx 17.1\%$.