Question

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

Answer 1

Avagadro's law gives the relationship between volume and number of moles of gas. 
At constant temperature and pressure , volume of gas is directly proportional to number of moles of gas. 
V / n = k
where V - volume, n - number of moles , k - constant 
$\frac{V1}{n1} = \frac{V2}{n2}$
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 - N
after doubling - 2N
initial volume - 2.10 L
final volume - V
substituting in the equation 

$\frac{2.10L}{N} = \frac{V}{2N}$
V = 4.20 L
new volume is 4.20 L