A cylinder of fixed capacity of 44 . 8 litres contains helium gas...

At STP volume occupied by one mole of a gas is $22 . 4L$. Therefore, number of moles are, $n = \dfrac{44 . 8}{22 . 4} = 2mol$.

The process is isochoric process as the volume of the cylinder remains fixed and helium is a monatomic gas. Therefore, amount of heat needed will be,

$Q = n Cv \DeltaT = \left(2\right)\left(\dfrac{3}{2}R\right)\left(20\right) = 3 \times 8 . 3 \times 20 = 498J$