We can answer this proble by using the ideal gas equation,
PV = nRT.
First we have to get the number of molecular weight of the substance using the first condition of the gas.
PV = nRT n = mass/molecular weight = M/MW
PV = MRT/ MW
M = 9.1 g
R = 0.08206 atm-L/mole-K
T = 23 + 273.15 = 296.15 K
P = 785/760 = 1.0329 atm
V = 15L
MW = MRT/PV
= (9.1g)(0.08206atm-L/mole-K) (296.15K)/(1.0329atm)(15L)
Now we can get the density of the subtance at STP.
At STP, T = 273.15, P = 1atm.
density= P MW/ RT
= (1atm)(14.2737 g/mole)/(0.08206atm-L/mole-K)(273.15K)
density = 0.6368 g/L = 0.64 g/L
Hope this helps :)