Answer:
2H2(g) + O2(g) --------> 2H2O(g). ∆H= -242KJmol-1
Explanation:
A thermochemical equation is a balanced chemical reaction equation that includes the physical states of all reactants and products and the energy change.
As shown above, hydrogen reacted with oxygen in the gas phase to form water, an energy of 242KJ was evolved and we are required to put this in a thermochemical equation using the least coefficients.
Hydrogen reacts with oxygen to form water in the gas phase in a least ratio of 2:1 as shown in the answer.
The term 'energy evolved' in the question means that the reaction is exothermic and energy is evolved. Hence ∆H is negative as shown.
Find the molar mass of Fe_2O_3
2Fe = 2*56 = 112
3O = 3* 16 = 48
Molar Mass = 160 grams
moles= given mass / molar mass
moles = 28 / 160
moles = 0.175 <<<< answer
Mass of chlorine =
(given)
Molar mass of chlorine =
Now, number of moles is defined as the ratio of given mass in gram to the molar mass of the compound.
Number of moles = 
Number of moles of chlorine =
= 
Now, according to significant figure rules, the least number of significant figure in the question identifies the number of significant figures in the solution.
Thus, in question, both digit have four significant figures, therefore, the answer must be in four significant figures.
Hence, number of moles of chlorine = 