Explanation:
The activation energy of a chemical reaction is the amount of energy that must be added to go from the energy level of the reactants to the energy level of transition state.
To do this problem it is necessary to take into account that the heat given by the unknown substance is equal to the heat absorbed by the water, but considering the correct sign:

Clearing the specific heat of the unknown substance:

Answer:

Explanation:
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In this case, since the combustion of butane is:

Thus, since the molar mass of butane is 58.12 g/mol and that of carbon dioxide is 44.01 g/mol, we obtain the following mass of CO2 product:

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Answer:- pH is 2.14.
Solution:- Nitrous acid,
is a weak acid so first of all we solve for
and then figure out the pH.
the equation is written as:

Initial concentration for the acid is given as 0.120 M. Let's say the change in concentration is x. Then the equilibrium concentrations would be as:

= 
= 
Ka for nitrous acid is
and the equilibrium expression for this would be written as:
![Ka=\frac{[H_3O^+][NO_2^-]}{HNO_2}](https://tex.z-dn.net/?f=Ka%3D%5Cfrac%7B%5BH_3O%5E%2B%5D%5BNO_2%5E-%5D%7D%7BHNO_2%7D)
Let's plug in the values in it.

To make the calculations easy we could ignore
for the bottom and the expression becomes:

On cross multiply:

On taking square root to both sides:

So, ![[H_3O^+]=7.3*10^-^3M](https://tex.z-dn.net/?f=%5BH_3O%5E%2B%5D%3D7.3%2A10%5E-%5E3M)
Now we could calculate the pH using the pH formula:
![pH=-log[H_3O^+]](https://tex.z-dn.net/?f=pH%3D-log%5BH_3O%5E%2B%5D)

pH = 2.14
So, the pH of 0.120M nitrous acid is 2.14.