Answer:
Option (e) should be discarded.
Explanation:
The given set of data is said to be precise if the values are close to each other. In this problem, a chemistry student is experimentally determining the boiling point of bromine.
In this case, all values are close to each other but option (e) i.e. 56.3° should be discarded to make his data precise.
Because 25% is half of 0.5 and the half life of
is given as 5,715, the time it takes to reach this amount should just be double the half life: 11430 years.
However, in other cases the problem may not have easy numbers, so below is a more mathematical approach.
The half life formula is
.
Solving for t gives:

Given information:

Plug in given values and solve for t.

With correct significant figures, this would be 11000 years.
2C₃H₇OH + 9O₂ = 6CO₂ + 8H₂O
a₀=2
a₁=9
a₂=6
a₃=8
Answer:
ΔH°f C₂H₅O₂N(s) = -537.2kJ
Explanation:
Based on the reaction:
4 C₂H₅O₂N(s) + 9O₂(g) → 8CO₂(g) + 10H₂O(l) + 2N₂(g)
ΔHrxn = ΔH°f products - ΔH°f reactants.
As:
ΔH°fO₂(g) = 0
ΔH°fCO₂(g) = -393.5kJ/mol
ΔH°fH₂O(l) = -285.8kJ/mol
ΔH°fN₂(g) = 0
The ΔHrxn is:
ΔHrxn = (8×-393.5kJ/mol + 10×-285.8kJ/mol) - (4×ΔH°fC₂H₅O₂N(s)) = -3857kJ/mol
-6006kJ/mol - (4×ΔH°fC₂H₅O₂N(s)) = -3857kJ/mol
-4×ΔH°fC₂H₅O₂N(s) = 2149kJ/mol
ΔH°fC₂H₅O₂N(s) = 2149kJ/mol / -4
<h3>ΔH°f C₂H₅O₂N(s) = -537.2kJ</h3>
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