Answer
(a) 0.122 mol
(b) 0.145 atm
Work Step by Step
For (a):
We know that 4 g of He make 1 mol.
$\implies$ 1 g of He is $\frac{1}{4}$ mol.
$\implies (4.86 \times 10^ {-4}) $ g of He make $\frac{4.86 \times 10^-4}{4} = 0.122$ mol.
For (b):
Using $pV = nRT$,
$p = \frac{nRT}{V} = \frac{(0.122)(8.3145)(291)}{20 \times 10^{-3}} = 1.47 \times 10^4 Pa = 0.145$ atm.
