Question

NaCl has a mc032-1.jpgHfus = 30.2 kJ/mol and a mc032-2.jpgHvap = 171 kJ/mol. What mass of NaCl releases 452 kJ of heat as it changes into a liquid?
Use mc032-3.jpg.
2.64 g
22.1 g
155 g
875 g

Answer 1

Answer:

C

Explanation:

155

Answer 2

Answer: option 3) 155 g

Explanation:


1) Releasing energy means that it changed from gas to liquid (or liquid to solid, but it is told that the NaCl changed into liquid).


That is because molecules in gas state are more energetic than in liquid state (molecules in gas state have more kinetic energy than molecules in liquid stated).

2) So, that means that the corresponding process is condenstation, and you have to use the latent heat of vaporization which is told to be 171 kJ/mol.

3) Calculations: 

i) number of moles, n:
452 kJ = n × 171 kJ/mol ⇒ n = 452 kJ / 171 kJ/mol = 2.643 moles

ii) mass:
mass = n × molar mass = 2.643 moles × 58.443 g/mol = 154.46 g ≈ 155 g