How do I set up these problems?

Question

An oxide of nitrogen contains 30.45 mass % N.
(a) What is the empirical formula of the oxide?

(b) If the molar mass is 90 +/- 5 g/mol, what is the molecular formula?

Calculate the number of lithium ions in 1.25 g lithium sulfate.

Thank you

Answer 1

If the N = 30.45%, then the O must be 69.55%

Assume you have 100 grams and it ieasy to see the grams of each in that sample.

Divide by the molecular mass of wach element and you get moles. Convert to a ratio of whole number s and you get the EF.

Compare the mol mass of the EF to 90 and you get the MF.

Your second question requires to multiply the moles by 2 and then by Avogadros number