Explain why the mole is used as a counting unit for mass

It isn't.  

Explanation:

It is a counting unit for "things" — atoms, molecules, ions, etc. —  and these things all have different masses.

For example, a chemist may know that one atom of Cu reacts with one atom of S to form one molecule of CuS.

Cu + S ⟶ CuS

She knows that 1 mol of Cu (6.022 × 10²³ atoms of Cu) has a mass of 63.55 g and 1 mol of S (6.022 × 10²³ atoms of S) has a mass of 32.06 g.

By measuring  out 63.55 g of Cu and 32.06 g of S, she knows that there are just enough atoms of each element to react with each other.

Thus, the mole has enabled her to count atoms.

answer:

the answer is 6 valence electrons.

explanation:

valence is the # of electrons on the outer most shell of an atom.

oxygen has 6.

although, the valency of oxygen is 2, because it only needs to make 2 covalent bonds, or gain 2 electrons, to achieve the stable octet configuration of a noble gas.

that's it!

if you need further assistance, let me know: )

~ trueboss72

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.