KNO3 dissociates into K+ and NO3-. NO3- is a conjugate base of a strong acid. Strong acids have very weak conjugate bases; this is why HF is a weak acid. So it's as if putting a very weak base into solution.
The same couldn't be said about F-, NH4+, S2-, CO3-2...assuming "Na2Co3" is not a cobalt compound. Carbonate formula is CO3 not Co3, as Co is cobalt. If you wrote this on a test you'd lose points.