What is the total mass of 3.01 x 10^23 atoms of helium gas?

2 g

Moles of atom =  atoms / 6.022 × 10^23  

Therefore; number of moles;

=(3.01 ×10^23) / (6.022 × 10^23)

= 0.5 moles

But;

Mass = moles of atom × Molar Mass of Helium  

Thus total mass of helium:

= 0.5 × 4

= 2 g

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.