607 m/s
First, let's calculate the mass of a helium atom. You do that by dividing the mass of 1 mole of helium atoms by avogadro's number. So
4 / 6.0221409e23 = 6.6422e-24 g = 6.6422e-27 kg
The equation for kinetic energy is
E = 0.5 M V^2
And we have 1.224e-21 J, which is 1.224e-21 kg*m^2/s^2
So substitute the known values into the equation for kinetic energy and solve for V
1.224e-21 kg*m^2/s^2 = 0.5 6.6422e-27 kg V^2
1.224e-21 kg*m^2/s^2 = 3.32108e-27 kg V^2
368555 m^2/s^2 = V^2
607 m/s = V
