What is the molar mass of helium in kg/mol?

Molar mass of helium is 4.g.mol⁻¹

But you wanted in unit kg.mol⁻¹

So the molar mass of helium is 4g.mol⁻¹ × 10⁻³ k .g ⁻¹

= 4 × 10⁻³ kg.mol⁻¹