As with any combustion reaction, the products of combusting a hydrocarbon fuel (

Question

As with any combustion reaction, the products of combusting a hydrocarbon fuel (CxHy) with oxygen (O2) are carbon dioxide (CO2) and water (H2O). A mass of 17.22 g for an unknown fuel was combusted in a reaction vessel containing an unknown amount of oxygen. At the end of the reaction, there still remained 17.20 g of the fuel as well as 0.0327 g of water and 0.0599 g of carbon dioxide. The oxygen was completely consumed during the reaction. How many molecules of oxygen gas were initially present in the reaction vessel? Express the number of molecules to three significant digits.

Explanation / Answer

The balanced reaction will be

CxHy + O2 ------- xCO2 + (y/2)H2O

Number of moles of CO2 = Mass/molar mass = 0.001361 moles

Number of moles of H2O = Mass/molar mass = 0.0018 moles

Number of moles of oxygne gas = Number of moles of CO2 + Number of moles of H2O/2

=> 0.001361 + 0.001816/2

=> 0.002269 moles

Molecules of O2 gas present = number of moles * avogadro number = 0.002269 * 6.023 * 10^(23)

=> 1.37 * 10^(21) molecules

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