D. How the pull of gravity has changed.
Answer:
Highest boiling point - 0.43 m Urea
Second highest boiling point - 0.20 m NiSO4
Third highest boiling point - 0.19 m NH4I
Lowest boiling point - 0.17 m NH4NO3
Explanation:
We know that;
ΔT = kb m i
Where;
ΔT = boiling point elevation
kb = boiling point constant
m = molality of the solution
i = Van't Hoff factor
For NiSO4 , NH4I and NH4NO3 , the Van't Hoff factor, i = 2
But for Urea, the Van't Hoff factor, i = 1
We also have to consider both the values of the molality and Van't Hoff factor , knowing that a higher molality and a higher Van't Hoff factor leads to a higher ΔT and consequently a higher boiling point.
This facts above account for the arrangement of substances shown in the answer.
<u>Answer:</u> The correct answer is Option A.
<u>Explanation:</u>
Standard enthalpy of formation is the change in enthalpy of one mole of a substance present at the standard state that is 1 atm of pressure and 298 K of temperature. The substance is formed from its pure elements under the same conditions.
We are given a chemical compound having chemical formula 
This compound is formed by the combination of calcium, nitrogen and oxygen elements.
The chemical equation for the formation of
from the components in their standard states follows:

Hence, the correct answer is Option A.