What is the empirical formula of the hydrocarbon?

Empirical Formula of hydrocarbon by Combustion Analysis?

• A hydrocarbon burns completely, producing 7.2 g water and 7.2 liters of CO2 at standard conditions. What is the empirical formula of the hydrocarbon? Since we're not given the mass of the hydrocarbon to begin with, how do I go about solving this?

• Answer:

  You are given the mass of the hydrocarbon, however, it is hidden in the combustion data.. Hydrocarbon + O2 >>> H20 + CO2 The goal is to find out how many moles of "carbon" are in the CO2 and how many moles of "hydrogen" are in the water. Once you know these numbers, then you should be able to get the empirical formula. Since the hydrocarbon is " burned completely" , then you can do this problem. If it wasn't and this was the only information you had, then you couldn't solve it.. Let's start with the number of moles of carbon.. You have 7.2 liters of CO2 and standard conditions. Here, you would use your gas law equation.. PV=nRT You want to know how many moles of carbon dioxide are present , so solve for "n" n=PV/RT Plugging in the numbers for P,V,R and T. Standard conditions are 100kPa (or 1 bar) @ 273K (or 0°C).. http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure n= (100kPa)*(7.2 Liters)/(8.3145)*(273K) n=0.317 moles of CO2 Since the number of moles of CO2 is the same as the number of moles of Carbon (C), then the number of moles of C is also 0.317. Now, let's look at hydrogen which can be found from the H20 data. You have 7.2 grams of H2O which you can use to find the number of moles of H20. The atomic weight of H2O is 18. Therefore, the number of moles of H20 in 7.2 grams is... 7.2 g / 18 g/mole = 0.4 moles of H20 Since there are 2 moles of H for every mole of H2O, you have 0.8 moles of H based upon the ratio of C to H.. 0.3 / 0.8 the empirical formula is C3H8 which is propane.