If the number of moles of a gas initially contained in a 2.10 l vessel is doubled, what is the final volume of the gas in liters? (assume the pressure and temperature remain constant.) none of the above 8.40 4.20 1.05 6.30

Avagadro's law gives the relationship between volume of gas and number of moles of the gas. When the temperature and pressure are constant, volume is directly proportional to number of moles of gas.

V -volume and n - number of moles of gas.
parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation 
initial number of moles n1 - N
n2 - 2N as the number of moles are doubled 
V1 - initial volume - 2.10 L
V2 - final volume - V
substituting the values in the equation 

V = 4.20 L 
final volume is 4.20 L