Answer:
The standard enthalpy of formation of methanol is, -238.7 kJ/mole
Explanation:
The formation reaction of CH_3OH will be,
The intermediate balanced chemical reaction will be,
..[1]
..[2]
..[3]
Now we will reverse the reaction 3, multiply reaction 2 by 2 then adding all the equations, Using Hess's law:
We get :
..[1]
..[2]
[3]
The expression for enthalpy of formation of 
will be,
The standard enthalpy of formation of methanol is, -238.7 kJ/mole
Answer:
The weights of all elements are always compared to the Carbon-12.
Explanation:
The weights of all elements are always compared to the Carbon-12 because the mass of carbon is 12 which is the exactly the sum of protons and neutrons.
Oxygen was also considered the standard for some time but later this stander was rejected because in natural O¹⁷ and O¹⁸ were also present and this create the two different atomic mass tables.
AMU:
Atomic mass unit is define as the 1/12 the mass of an atom of carbon-12.
C12 has six neutron and six protons in the nucleus.
This unit is used to express the masses of atoms. We know that masses of atoms are very small and we do not have any such type of balance that can measure very small quantity. So that is way we use this scale to measure small quantity. For example, according to this scale
relative atomic mass of hydrogen is 1.008 amu
relative atomic mass of oxygen is 15.999 amu
relative atomic mass of uranium is 238.0289 amu
relative atomic mass of chlorine is 35.453 amu
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