Which weighs more 5moles of sodium nitrate or 4 moles of carbon dioxide?

I'm assuming that your instructor is using the terms mass and weight interchangeably.

First determine the molar mass of each compound by multiplying the molar mass of each element by its subscript and add them together. The molar mass of an element is its atomic weight on the Periodic Table in grams.

Molar mass of sodium nitrate (NaNO3)

(1 atom Na x 22.98970g/mol Na) + (1 atom N x 14.0067g/mol N) + (3 atoms O x 15.9994g/mol O) = 84.9946g/mol NaNO3

Molar mass of carbon dioxide (CO2)

(1 atom C x 12.0107g/mol C) + (2 atoms O x 15.9994g/mol O) = 44.0095g/mol CO2

Now multiply the molar mass for each compound by the number given and compare the results to determine which has the greater mass.

5mol NaNO3 x 84.9946g/mol NaNO3 = 424.973g NaNO3

4 mol CO2 x 44.0095g/mol CO2 = 176.038

So 5 moles of sodium nitrate has a mass greater than 4 moles of carbon dioxide.