Finding the quantity of heat

Question

What quantity of heat, in kJ, is required to convert 17.0 g of ethanol (C₂H₅OH) at 23.0 °C to a vapor at 78.3 °C (its boiling point)? (specific heat capacity of ethanol = 2.46 J/g • °C; ∆Hvap = 39.3 kJ/mol)

Answer 1

step 1: heat required to raise temperature from 23.0º to 78.3º:

q = mC∆T

q = (17.0 g)(2.46 J/gº)(55.3º) = 2313 J = 2.313 kJ

Step 2: heat required to convert the sample from liquid to vapor at 78.3º (phase change):

q = m∆H_vap

q = (17.0 g)(39.3 kJ/mol)(1 mol/46.07 g) = 14.50 kJ

Step 3: Sum the heat values and report to 3 significant figures:

q = 2.313 kJ + 14.50 kJ

q = 16.8 kJ