Number of moles can be calculated from a given volume. example:
Calculate the number of moles of gas present in the container of volume 10lit at 300K. If the manometer containing glycerine shows 5m difference in level as shown in the diagram.
Volume of container=10L
Density of glycerine=2.72g/ml
Density of Mercury= 1.36g/ml
Now, The height difference by glycerine=5m
The height by mercury=xm
We know that
P=ρgh [ρ=density, h=height, g acceleration due to gravity ]
Pressure of gas Pgas=0.76dg+1dg
=1760mm of Hg
1 atm= 760mm Hg
⇒1760mm Hg=1760/760 atm
Now, for moles of gas
We know, PV=nRT
No. of moles of gas≈0.94 moles.