Calculate your steam's pressure in kilopascals (kPA) using the Ideal Gas equation, PV = nRT, where P = pressure, V = volume in liters, n = number of moles, R is the Universal Gas Constant (8.3145 J/mol K) and T = temperature in Kelvin (K). For example, if your problem tells you that you have 1.81 moles of steam which has a volume of 2.58 liters as it escapes its container at a temperature of 297.15 K, you can calculate its pressure as follows: P(2.58) = (1.81)(8.3145)(297.15), or 2.58P = 4471.88, or P = 4471.88/2.58 = 1733.29 kPA.
Convert your pressure from kilopascals to bars and round to the nearest whole bar. One bar is equal to 100 kilopascals, so simply divide your pressure in kilopascals by 100. The example steam has a pressure of 1733.29 kPa, or 17.33 bars. Round this down to 17 bars.
Print your saturated steam table if you have not already done so and locate the line that contains your pressure. Find your steam's heat of vaporization under a column bearing the same name. Steam at a pressure of 17 bars, for instance, has a heat of vaporization of 1921.55 kilojoules per kilogram (kJ/kg) or 458.95 kilocalories per kilogram (kcal/kg) .