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 245.0 g of water, initially at 31.0 °C to a calorimeter and adds [your birth MONTH].31 g of NH4Cl (s), stirring to dissolve. After the NH4Cl dissolves completely, the temperature of the solution is [first two digits of your student ID].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. (10 points)

(b) Calculate the ΔHsoln for NH4Cl, in kJ/molrxn. Include units and proper sign in your answer. Show all work. (10 points)

(c) Is the dissolving of NH4Cl endothermic

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) $\Delta H$ = q = - 31,899 joules

(c) No , exothermic 's , since

$\Delta H$ is negative