Answer : The enthalpy of formation of 
is, -812.4 kJ/mole
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 formation of will be,
The intermediate balanced chemical reaction will be,
(1) 
(2) 
(3) 
(4) 
Now adding all the equations, we get the expression for enthalpy of formation of will be,
Therefore, the enthalpy of formation of is, -812.4 kJ/mole
GaBr3
Gallium=Ga
Bromine= Br
Bromide=Br3
A sample of a compound contains 60.0 g C and 5.05 g H.
divide by molar mass of C(12) and H(1) to get molar ratio
C: 60/12=5 and H: 5/1=5
so C:H=5:5=1:1
total molar mass=78
divide by 1C+1H to find the formula: 78/(12+1)=78/13=6
compound is C6H6
Answer:
1.635 M
Explanation:
Given:
10 mL of 20 volumes Hydrogen Peroxide
Here,
20 volumes of Hydrogen Peroxide means that on decomposition of 1 mL of H₂O₂ 20 mL of O₂ is obtained
also,
means 1 dm³ of H₂O₂ solution produces 20 dm³ oxygen
Now,
at 298K and 1 atm
20 dm³ oxygen = 
moles
or
= 0.817 moles
also,
2H₂O₂ → 2H₂O + O₂
thus,
1 dm³ of solution must contain 2 × moles of O₂ as moles of H₂O₂
thus,
Number of moles of H₂O₂ = 2 × 0.817
or
Number of moles of H₂O₂ = 1.635 moles
Hence,
For 20 volume hydrogen peroxide is 1.635 M
