A chemical equation does not give information about the following:
- It usually does not give the "state of the substances". There are three states: Solid(s), liquid(q) and gas(vap).
- The chemical equation does not show whether it is complete or incomplete.
- The "speed of the reaction" is not mentioned.
- The "concentration of the substance" whether it is diluted or concentrated is not mentioned.
- The "rate of the reaction", temperature, catalyst, pressure etc is not mentioned. These can be mentioned "above or below the arrow".
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Answer:

Explanation:
In this problem, the temperature stays constant. The volume and pressure change, so we use Boyle's Law. This states that the pressure of a gas is inversely proportional to the volume. The formula is:

Now we can substitute any known values into the formula.
Originally, the gas has a volume of 25.0 liters and a pressure of 2.05 atmospheres.

The volume is decreased to 14.5 liters, but the pressure is unknown.

Since we are solving for the new pressure, or P₂, we must isolate the variable. It is being multiplied by 14.5 liters and the inverse of multiplication is division. Divide both sides by 14.5 L .


The units of liters cancel.



The original values of volume and pressure have 3 significant figures, so our answer must have the same.
For the number we found, that is the hundredth place.
The 4 in the thousandth place (in bold above) tells us to leave the 3 in the hundredth place.

The new pressure is approximately <u>3.53 atmospheres.</u>
∆H ° rxn =-2855.56 kJ
<h3>Further explanation</h3>
Given
ΔHf CO₂ = -393.5 kJ/mol
ΔHf H₂O = -241.82 kJ/mol
ΔHf C₂H₆ = - 84.68 kJ/mol
Reaction
2C2H6(g) + 7O2(g) -> 4CO2(g) + 6H2O(g)
Required
ΔHrxn=
Solution
<em>∆H ° rxn = ∑n ∆Hf ° (product) - ∑n ∆Hf ° (reactants) </em>
∆H ° rxn = (4.-393.5+6.-241.82)-(2.-84.68)
∆H ° rxn = (-1574-1450.92)-(-169.36)
∆H ° rxn =-3024.92+169.36
∆H ° rxn =-2855.56 kJ
Heat needed=mcθ
=7×0.031×(42-17)
=5.425cal