Here's why:

* Gram-formula mass: This term refers to the mass of one mole of a substance. It's essentially the same as molar mass, but the term "gram-formula mass" is often used when discussing ionic compounds.

* NO₂: This is the chemical formula for nitrogen dioxide.

To calculate the gram-formula mass of NO₂, you need to add up the atomic masses of all the atoms in the molecule:

* Nitrogen (N): 14.01 g/mol

* Oxygen (O): 16.00 g/mol (and there are two oxygen atoms)

Therefore, the gram-formula mass of NO₂ is:

14.01 g/mol + (16.00 g/mol x 2) = 46.01 g/mol