An experimental automobile burns hydrogen for fuel. At the beginning of a test drive, the rigid 30.0-L tank was fi lled with hydrogen at 16.0 atm and 298 K. At the end of the drive, the temperature of the tank was still 298 K, but its pressure was 4.0 atm. (a) How many moles of H2 were burned during the drive? (b) How much heat, in kilojoules, was given off by the combustion of that amount of hydrogen?

(a) 14.7 mol

(b) - 4.20 kJ

Explanation:

First, we will find the initial moles of H₂ using the ideal gas equation.

P * V = n * R * T

16.0 atm * 30.0 L = n * (0.08206 atm. L/mol. K) * 298 K

n = 19.6 mol

The moles of H₂ after the reaction are:

P * V = n * R * T

4.0 atm * 30.0 L = n * (0.08206 atm. L/mol. K) * 298 K

n = 4.9 mol

The moles of H₂ that reacted are 19.6 mol - 4.9 mol = 14.7 mol

The heat of combustion for H₂ is - 286 kJ/mol. The amount of heat released when 14.7 moles of H₂ are burned is:

14.7 mol * (-286 kJ/mol) = - 4.20 * 10³ J = - 4.20 kJ