You are asked to calculate ?H when 0.1 kg of steam is heated at a constant pressure of 0.1 MPa from 373.15 K to 423.15 K. You remember learning that the heat capacity of water is about 4.19 kJ/kg•K, so you multiply 0.1 kg by 50 K by 4.19 kJ/kg•K and get an answer of 20,950 joules. Which of the following is the best assessment of your answer?

Question

A. Your calculation is accurate, but you should round the answer to 21,000 joules, since your heat capacity only had three significant figures and 20,950 joules has four.
B. This is a reasonable estimate of ?H, but it would have been more accurate to use the data in the steam tables than to use MCP?T.
C. This is a reasonable estimate of ?H, but it assumes CP is constant with respect to temperature. You can get a more accurate answer by accounting for the temperature dependence of CP.
D. This is not a reasonable estimate of ?H. 4.19 kJ/kg•K is a heat capacity of liquid water and you cannot assume water vapor has the same heat capacity.
E. This is not a reasonable estimate of ?H. You should have used CP* = (5/2)R.

Answer 1

A. Incorrect. You are correct about the significance of the number, but there is a flaw in the calculation.
B. Incorrect. The steam tables are preferred to MCP?T, but this is not a reasonable estimate.
C. Incorrect. You could get a more “refined” answer by integrating the heat capacity, but it would still be wrong.
D. Correct. The heat capacity of water vapor is less than half of the heat capacity of liquid water.
E. Incorrect. That formula only has any validity for monatomic ideal gases. Water is not monatomic.