I did a lab in chemistry in which 15-mL jumbo Beral pipets with acetone was heated in boiling water until all of the liquid evaporated. Then the pipets were placed into room temperature water so that the evaporated liquid turns into condensed vapor. The molar mass was later found using mass, temperature, pressure, and volume.

Question

I did a lab in chemistry in which 15-mL jumbo Beral pipets with acetone was heated in boiling water until all of the liquid evaporated. Then the pipets were placed into room temperature water so that the evaporated liquid turns into condensed vapor. The molar mass was later found using mass, temperature, pressure, and volume.

What effect would vapor condensation in the neck of the pipets (not the bulb, but the longer part of the pipet) affect the mass of the pipets?
(To me, it doesn't seem like there would be too much difference on whether the vapor condensation occured in the neck or the bulb since the mass of the pipet would still be the same in either place. However, I'm guessing that there should probably be some sort of error when it's condensed in the neck and not the bulb?)

Answer 1

When vapor condenses in the neck of the pipets instead of the bulb, it could have an effect on the mass of the pipets. The condensed vapor in the neck would add additional mass to the pipets, which would not have been accounted for in the initial measurement of their mass.

To understand this conceptually, let's consider the ideal gas law, which states that PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the ideal gas constant, and T is the temperature in Kelvin.

In your experiment, the initial mass, the volume of the pipet, the temperature, and the pressure were used to calculate the molar mass. However, if vapor condenses in the neck of the pipet, it means that the volume of the gas phase has decreased. This would lead to an underestimation of the moles of gas in the pipet when using the ideal gas law equation to calculate the molar mass.

Since the molar mass is calculated by dividing the mass of the sample by the number of moles, a lower number of moles due to condensation in the neck would result in a slightly higher calculated molar mass than the actual molar mass.

Therefore, it's important to ensure that vapor condensation occurs primarily in the bulb rather than the neck to minimize any potential errors in the molar mass calculations.