Question

how is the oxide of arsenic worked out when its in its gas state


please help im flumuxed

Please clarify your question? What do you mean by "worked out?

the question states
determine the molecular formular of the oxide of arsenic when it is in its gaseous state

evidence
arsenic heated at 650c pressure of 890mm hg the 0.927g sample of oxide and vol 194cm3

Use PV = nRT
p is pressure in atmospheres. You have pressure of 890 mm Hg and that is 890/760 = ?? atmospheres.
V is in liters. Your 194 cc = 0.194 L
n = number of mols. Solve for this.
R = constant of 0.08206 L*atm/mol*K
T = 650 C = 273+650 = ?? Kelvin.

Solve for n, THEN
you know mols = grams/molar mass.
You will know mols and grams. solve for molar mass of the oxide.