Answer to Question #164856 in General Chemistry for dgf

Question

A student investigates the enthalpy of solution, ΔHsoln for ammonium chloride, NH4Cl. In

addition to the salt, the student has access to a calorimeter, a balance with a precision of

±0.1 g, and a thermometer with a precision of ±0.1°C.

To measure ΔHsoln for NH4Cl, the student adds 904.0

g of water, initially at 31.0 °C to a calorimeter and adds 155 g of NH4Cl (s), stirring to dissolve. After the NH4Cl dissolves completely, the temperature of the solution is 90.0 °C. Assume the calorimeter was perfectly insulated and the specific heat of the

solution was 4.2 J/(g·K).

(a) Calculate the q for the dissolving of NH4Cl in water. Include units and proper sign in

your answer. Show all work.

(b) Calculate the ΔHsoln for NH4Cl, in kJ/molrxn. Include units and proper sign in your

answer. Show all work.

(c) Is the dissolving of NH4Cl endothermic or exothermic? Justify your answer.

Answer 1

(a) m = 245g

T2 = 0⁰C = 273K

T1 = 31⁰C = 304K

C = 4.2 J/(g•K)

$Q = mC ΔT$

Q = 245×4.2×(273-304)

Q = 245×4.2×(-31)

Q = - 31,899 Joules

(b) $\Delta HΔH$ = q = - 31,899 joules

(c) No , exothermic 's , since

\Delta HΔH is negative