how to calculate 2.0 mol of NaNO3?

I suppose that ypu are asking "how many oxygen atom are there in 2.0 moles of NaNO₃".

1 mole of NaNO₃ contains 6.022 × 10²³ formula units of NaNO₃.
1 formula unit of NaNO₃ contains 3 O atoms.

No. of O atoms = 2.0 × (6.022 × 10²³) × 3 atoms = 3.6 × 10²⁴ atoms

that's like asking
.. "how to calculate the number 2.0 dozen apples"

does that make any sense to you?

2.0 mol * 84.9947 g NaNO3/mol = 170 g NaNO3.

Yes