Calculating Pressure Exerted on A Gasoline Column

[JN Morris]

I am requesting for another set of eyes to double check my work, I believe my answer is correct but I am not sure. I also do not understand why 0.75 is the specific gravity of gasoline in this equation when it actually can range from 0.71-0.77

1. Homework Statement

Calculate the pressure exerted on a 26-ft column filled with 13 ft. of gasoline. The vapor pressure of gasoline at 100°F is 12 psi.

Homework Equations

Height x 0.433 x specific gravity (according to the textbook) = pressure.

The Attempt at a Solution

26 ft. x 0.433 x 0.75 = 8.4435 psi + 12 psi = 20.4435 psi

 

[DEvens]

Height x 0.433 x specific gravity (according to the textbook) = pressure.

Ohhhhh... Magic numbers make my stomach hurt.

Presumably the 0.433 is a unit converter that tells you the pressure, in psi, of 1 foot of water. It seems about right. If that is what the text tells you, then so good so far. Except our arithmetic seems a bit wonky. Check your multiplication please.

The column is 26 feet, and you have used that number in your answer. But the column is 26 feet high, not the gas. What if there was only a tenth of an inch of gas in the column? What would the pressure be then?

For completeness, your equation should include the vapor pressure, since you included that later.

 

[insightful]

Calculate the pressure exerted on a 26-ft column filled with 13 ft. of gasoline. The vapor pressure of gasoline at 100°F is 12 psi.

Is the column open to the atmosphere? If not, anything other than gasoline vapor above the liquid level? Is it asking for the pressure at the bottom of the column (probably, but not stated)? As implied above, your answer is not correct.