Answer to Question #279242 in General Chemistry for Emre

Question

Q10. In a Styrofoam cup, a 1.80 g sample of NaCl is added to 32.0 g H2O and stirred until

NaCl completely dissolves. The temperature of the solution drops from 25.4 to 23.1 °C.

(a) Is the process endothermic or exothermic?

(b) What is the heat of solution of NaCl expressed in kilojoules per mole of NaCl?

(Specific heat of water: 4.18 J/g °C)

Answer 1

(a) The process is endothermic, because temperature of the solution droped (the heat was taken).

(b) q = m x c x ∆T

q is the heat lost by the water; m is the mass of the water; c is the specific heat of water; ΔT  is the change in temperature, defined as the difference between the final temperature and the initial temperature of the sample.

We can approximate the mass and the specific heat of the solution to be equal to those of the pure water sample.

q = 32.0 g x 4.18 J/g °C x (23.1°C - 25.4°C) = -307.65 J

∆H = 307.65 J

Convert the mass of sodium hydroxide to moles by using the compound's molar mass

1.80 g x 1 mol NaCl / 58.44 g = 0.03 mol NaCl

∆H = 1 mole NaCl x 307.65 J / 0.03 mol = 10 255 J/mol = 10.26 kJ/mol