What amount of heat (in kJ) is required to convert 19.1 g of an unknown liquid (MM = 83.21 g/mol) at 19.2 °C to a gas at 93.5 °C? (specific heat capacity of liquid = 1.58 J/g･ °C;

Question

specific heat capacity of gas = 0.932 J/g･ °C; ∆Hvap = 22.5 kJ/mol; normal boiling point, Tb = 57.3 °C)

Answer 1

step 1. liquid at 19.2º to liquid at 57.3º

q = mC∆T

q = (19.1 g)(1.58 J/gº)(38.1º) = 1150 J

Step 2. liquid @ 57.3 to gas @ 57.3 (phase change)

q = m∆Hvap

q = (19.1 g)(22.5 kJ/mol x 1 mol / 83.21 g ) = 5.165 kJ = 5165 J

Step 3. gas @ 57.3º to gas @93.5º

q = mC∆T

q = (19.1 g)(0.932 J/gº)(36.2º) = 644 J

Total amount of heat needed = 1150 J + 5165 J + 644 J = 6959 J or 6.96 kJ