Question

Calculate the formula wright of a 2.50 g sample of gas whose volume is 1.40l at STP.

Answer 1

Use PV = nRT and solve for n = number of mols; then n = grams/molar mass. You know n and grams, solve for molar mass (formula weight).

OR, if you remember that a gas at STP occupies 22.4 L, then
1.40 L/22.4 = n, then n = grams/molar mass.