the apparatus in the figure below is used for the electrolysis of water. hydrogen and oxygen gas are collected in the two inverted burets. an inert electrode at the bottom of the left buret is connected to the negative terminal of a 6 v battery, whereas the electrode in the buret on the right is connected to the positive terminal. a small quantity of sodium carbonate is added to speed up the electrolytic reaction.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.