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# A 100.-mL sample of liquid water is heated in a flask to boiling at 1 atm. As the water boils, some

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. 

As water changes from solid to liquid (melting) and liquid to gas (boiling or vaporization), the potential energy increases.

61 Compare the entropy of the H2O(l) to the H2O(g) that is formed. 

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. 

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)