# Pycnometer equation

hello!

On wiki, regarding specific gravity, it gives the derivation for the pycnometer equation for determining specific gravity. However, the first equation confuses me. It goes as follows:
The pycnometer, placed on a balance, will exert a force:
$$F = g(m_b - \frac{ \rho_a m_b}{ \rho_b})$$
The subscript b is for the bottle, and a is for air. rho is density and m is mass; g is the acceleration due to gravity
The force on the balance would be F = gm. The above equation suggests that the mass of the air displaced by the bottle needs to be subtracted from the mass of the bottle; so the mass used in the equation is the difference in mass between the bottle and the air that would occupy the space of the material used for the bottle. I don't understand why this is the case. Why does the mass of air that would occupy that space if the bottle wasn't there even matter? It seems irrelevant. Surely we need to consider the mass of the bottle, all of its mass.

Any help appreciated.

Borek
Mentor
You have to go back to Archimedes.

Last edited:
hello!

On wiki, regarding specific gravity, it gives the derivation for the pycnometer equation for determining specific gravity. However, the first equation confuses me. It goes as follows:
The pycnometer, placed on a balance, will exert a force:
$$F = g(m_b - \frac{ \rho_a m_b}{ \rho_b})$$
The subscript b is for the bottle, and a is for air. rho is density and m is mass; g is the acceleration due to gravity
The force on the balance would be F = gm. The above equation suggests that the mass of the air displaced by the bottle needs to be subtracted from the mass of the bottle; so the mass used in the equation is the difference in mass between the bottle and the air that would occupy the space of the material used for the bottle. I don't understand why this is the case. Why does the mass of air that would occupy that space if the bottle wasn't there even matter? It seems irrelevant. Surely we need to consider the mass of the bottle, all of its mass.

Any help appreciated.

The second term:

$$- \frac{ \rho_a m_b}{ \rho_b}$$

is the correction to the weight of the bottle for the buoyancy of air. In the most precise work, this must be determined using air temperature, pressure, and humidity. This can amount to a correction of a few parts per million.

Thanks for the responses. I have a really dumb question: if something is flat on the surface, how can the air "push up" underneath the bottle? Dumb question, I know, but I am trying to visualise where the pressure comes from! Thanks for the responses so far!