Please answer to two (2) questions among 4, 5 or 6, and leave the other one blan

Question

Please answer to two (2) questions among 4, 5 or 6, and leave the other one blank. Show your steps, how you cancel the units, and use the appropriate significant figure. 5. An experiment aims to determine the heat of dissolution (Delta H diss) of sodium nitrate salt. A student dissolves 15.3 g of NaNO3 salt in a coffee-cup calorimeter, and the temperature falls from 25.00 degree C to 21.56 degree C. The calorimeter constant was measured earlier. 1071 J/degree C. The dissolution equation is: a) Write the overall equation showing the heat balance of the system NaNO3 - calorimeter. Clearly indicate what constitutes the system, and what constitute the surroundings. b) Calculate the enthalpy change (Delta H diss) normalized for the dissolution of 1 mole of NaNO3 in water (the molar mass of NaNO3 is 84.99 g). c) Indicate if this reaction is endo- or exothermic, and justify based on the data or your calculations.

Explanation / Answer

(a)

DHsoln=heat of dissolution

Qsoln = Heat absorbed by NaNO3 solution

Qcal = Heat absorbed by the calorimeter

Qsurr = heat gained from surrounding= 0 as insulated

Qsoln+Qcal=Qsurr=O

(b)

DHsoln=heat of dissolution =Qsoln for one mole of NaNO3

Qcal =1071*(21.56-25) = -3684.24j

Qsoln = -Qcal =3684.24j for 15.3gm of NaNO3

For 1gm of NaNO3 Qsoln = 3684.24/15.3 = 240.8

1 mole of NaNO3 = 84.99gm

For 1 mole NaNO3 = 84.99*240.8 =20466j or 20.5kj/mole

(c)

It is endothermic as DHsoln is positive.

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