A0.529-g sample of gas occupies 125 ml at 60. cm of hg and 25°c. what is the molar mass of the gas?

Let's assume that the gas has ideal gas behavior. 
Then we can use ideal gas formula,
PV = nRT

Where, P is the pressure of the gas (Pa), V is the volume of the gas (m³), n is the number of moles of gas (mol), R is the universal gas constant ( 8.314 J mol⁻¹ K⁻¹) and T is temperature in Kelvin.

P = 60 cm Hg = 79993.4 Pa
V = 125  mL = 125 x 10⁻⁶ m³

n = ?

R = 8.314 J mol⁻¹ K⁻¹
T = 25 °C = 298 K

By substitution,
79993.4 Pa x 125 x 10⁻⁶ m³ = n x 8.314 J mol⁻¹ K⁻¹ x 298 K
                                          n = 4.0359 x 10⁻³ mol

Hence, moles of the gas = 4.0359 x 10⁻³ mol

Moles = mass / molar mass

Mass of the gas  = 0.529 g 

Molar mass of the gas = mass / number of moles
                                    = 0.529 g / 4.0359 x 10⁻³ mol
                                    = 131.07 g mol⁻¹

Hence, the molar mass of the given gas is 131.07 g mol⁻¹

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.