Question

why can't gravimetric analysis be used to determine the concentration of sodium nitrate (NaNO) in a solution?

I think the answer is because NaNO dissolves in water. Am I correct

Answer 1

I think your answer is the one that the "prof" wants but I might clarify a little.
Yes, NaNO3 is not easily determined both because most Na salts are soluble and most nitrate salts are soluble so there is no easy way to convert them to insoluble forms. Of course you have already pointed out that NaNO3 itself is quite soluble. However, I remember that sodium zinc uranyl acetate is insoluble and I have used that as a QUALITATIVE test for Na many years ago when I was a student. I found an article on the web that explains that under carefully controlled conditions Na can be QUANTITATIVELY determined if the amount to be determined is large enough. I don't have any first hand knowledge of its accuracy but in general I think you are on solid ground that quantitative determinations of Na are not easily done. K is another one of those that is difficult and for the same reasons.