The combustion of fuel in your car engine requires oxygen gas, which is supplied as air (21% oxygen molecules) into the engine. Consider a car that is using 100% ethanol, C2H5OH, as fuel. If your engine intakes 3.72 L of air per minute at 1.00 atm and 25ºC, what is the maximum volume of ethanol (0.789 g/mL) that can be burned per minute?

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Answer:

0.895 mL

Explanation:

The equation for the reaction can be written as:

C₂H₅OH + 3 O₂ ⇒ 2 CO₂ + 3 H₂O

Using ideal gas law

PV = nRT

where;

Pressure (P) = 1 atm

Volume (V) =  (3.2 L of air per minute)

General gas constant (R) =  0.082 atmL/molK

Temperature (T)=  25°C = 298.15 K

number of moles (n) = 0.192 mole

number of moles in 21% oxygen molecule will contain:

0.192 × 0.21 = 0.04032 moles

0.04032 moles O₂= [tex]\frac{1molC_{2}H_{5}OH}{3molesO_2} \frac{46,07 g}{1mol} \frac{1mL}{0,789g}[/tex]

= 0.895 mL of ethanol per minute

∴ the maximum volume of ethanol (0.789 g/mL) that can be burned per minute = 0.895 mL

The maximum volume of ethanol that can be burnt per minute is 0.617 ml.

Based on the given information,

• The volume of air is 3.72 L, pressure is 1 atm, and given temperature is 25 degree C or 273 +25 = 298 K.

• The number of molecules of oxygen in air is 21%.

Now the formula for ideal gas equation is,

PV = nRT

The number of moles of air = PV/RT (R = 0.0821 Latm/mol/K)

[tex]nair = \frac{1*3.72}{0.0821*298} \\nair = 0.15[/tex]

As the number of molecules of oxygen in air is 21%, now the number of moles of oxygen in air will be,

[tex]= \frac{21*0.15}{100} \\= 0.0317[/tex]

Now, the reaction of the burning of ethanol is,

C₂H₅OH + 3O₂ ⇔ 2CO₂ +3H₂O + heat

• 1 mole of C₂H₅OH requires 3 moles of oxygen to burn completely or it can be said that 1 mole of oxygen burns 1/3 moles of C₂H₅OH.

• Therefore, 0.0317 moles of O₂ burns = 0.0317/3 or 0.0106 moles of C₂H₅OH.

Now the molar mass of C₂H₅OH is 46 g/mol, and the mass of C₂H₅OH will be,

[tex]Mass of ethanol = No of moles * Molar mass\\Mass of ethanol = 0.0106 * 46 g/mol\\Mass of ethanol = 0.487 grams\\[/tex]

Now the density of C₂H₅OH is 0.789 g/ml, the volume can be determined by using the formula,

Volume = Mass/Density

[tex]Volume = \frac{0.487 g}{0.789 g/ml} \\Volume = 0.617 ml[/tex]

Thus, the maximum volume of ethanol that can be burnt per minute is 0.617 ml.

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