Exercise 7.23 - Enhanced with Feedback Part A You are planning a lecture demonst

Question

Exercise 7.23 - Enhanced with Feedback Part A You are planning a lecture demonstration to illustrate an endothermic process. You want to lower the temperature of 1800 mL water in an insulated container from 28 to 7.0° C. Approximately what mass of NH4 Cl(s) should you dissolve in the water to achieve this result? The heat of solution of NH4 Cl is +14.7kJ/mol NH4 Cl. Assume that the density of the dilute solution is the same as that of water. Express your answer to two significant figures and include the appropriate units. You may want to reference (D page ) while completing this problenm Section 7.1 | m= 1 V || Units atue Submit My Answers Give Up

Explanation / Answer

1] Calculate the heat loss in "lower the temperature of 1800 mL water in an insulated container from 28 oC to 7 oC.

This is a 'sensible' heat,
Q = M C DeltaT.

Where M = mass = volume x density = 1800 mL x 1 g/mL = 1800 g
C = specific heat for water = 4.184 J g-1oC-1
delta T = change in temperature = 28 oC – 7 oC = 21 oC

So, Q = (1800 g) x (4.184 J g-1oC-1) x (21 oC) = 158155.2 J

2] Divide the answer above by 14.7 kJ/mol NHCl to get mol NHCl

So, Moles of NH4Cl = (158155.2 J) / (14.7 kJ/mol)
                                = (158155.2 J) / (14700 J/mol)
                                = 10.76 mol

3] Multiply by molar mass NH4Cl

Molar mass of NH4Cl = 53.50 g/mol

So, Mass of NH4Cl = (53.50 g/mol) x (10.76 mol)
= 575.66 g

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