The average atomic mass includes all of the isotopes of that atom which exist in nature. Almost all elements have at least two isotopes which naturally occur.
Recall that isotopes are atoms with the same atomic number (protons), but a different number of neutrons. Since neutrons contribute the same amount of mass as a proton (1 amu), differing numbers of neutrons will change the mass of the atoms.
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Answer : The value of 
is, -103.4 kJ
Explanation :
According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.
According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.
The given main reaction is,
The intermediate balanced chemical reaction will be,
(1) 
(2) 
Now reversing and multiplying by 2 of reaction 1 and 2 then adding all the equations, we get :
(1) 
(2) 
The expression for will be,
Therefore, the value of is, -103.4 kJ
Answer:
P = 28.5 atm
Explanation:
PV = nRT => P = nRT/V
P = Pressure = ?
n = moles of sample = mass/molar mass = 26g/44g·mol⁻¹ = 0.619 mole CO₂
R = gas constant = 0.08206 L·atm/mol·K
T = temperature in Kelvin = (35 + 273)K = 308K
V = volume in Liters = 550 ml = 0.550 L
Pressure (P) = nRT/V
= (0.619 mole)(0.08206 L·atm/mol·K)(308K)/(0.550 L) = 28.5 atm
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It's only a small difference (103 degrees versus 104 degrees in water),
and I believe the usual rationalization is that since F is more
electronegative than H, the electrons in the O-F bond spend more time
away from the O (and close to the F) than the electrons in the O-H bond.
That shifts the effective center of the repulsive force between the
bonding pairs away from the O, and hence away from each other. So the
repulsion between the bonding pairs is slightly less, while the
repulsion between the lone pairs on the O is the same -- the result is
the angle between the bonds is a little less.
Hope this helps!
