Collecting gas over water with a eudiometer

[elements]

Homework Statement

When, using a eudiometer in a lab to collect gas over water, if you have to switch eudiometers is the final volume of water displaced equal to the water displaced in the first tube + the water displaced in the second, and would the pressure be calculated through the total height displaced?

i.e.

Collecting butene gas from a reaction of BuBr and KOH in a 50mL eudiometer tube. The first tube all of the 50mL of H2O is displaced and the second tube 23mL is displaced.

Homework Equations

Total Volume = Volume displaced in first tube + Volume displaced in second
Pressure = Pressure of both heights combined

The Attempt at a Solution

Total Volume of gas produced = 50mL + 23mL=73mL of gas produced
Pressure = 50cm+(50-23cm)=770mmH2O

 

[Borek]

is the final volume of water displaced equal to the water displaced in the first tube + the water displaced in the second

And what is the alternative?

 

[elements]

just the final volume of water displaced in the second eudiometer tube

 

[Borek]

So, what its the reasoning behind ignoring the first portion of gas collected?

Imagine using twice larger tube - should you get the same result, or a different one from the same experiment? Would you get the same result if you ignore the volume collected in the first tube?