Regents Chemistry Exam August 2022
A 100.-mL sample of liquid water is heated in a flask to boiling at 1 atm. As the water boils, some liquid water changes phase to water vapor. The equation below represents this change.
H2O(l) + heat → H2O(g)
60 Describe the change in potential energy of the water molecules that vaporize during
boiling. [1]
As water changes from solid to liquid (melting) and liquid to gas (boiling or vaporization), the potential energy increases.
Answer: Potential energy increases
61 Compare the entropy of the H2O(l) to the H2O(g) that is formed. [1]
Entropy represents disorder. What has more disorder liquid or gas? Gas has a higher entropy than liquid and liquid has a higher disorder than solid.
Answer: H2O(g) has a higher entropy that H2O(l)
62 Determine the mass of liquid water that vaporizes if 7700 joules of energy is absorbed by the H2O(l) at 100.°C. [1]
Let's take a look at Table T at the Reference Table. The questions is asking about vaporization of water. If we take a look at the heat equations we see q = mHv where q=heat, m = mass and Hv= heat of vaporization. Heat of vaporization of water is in Table B at the Reference Table and is 2260 J/g. Let's now plug everything in.
q = mHv
7700 joules = m (2260 J/g)
Answer: m = 3.4 g