Calculate mixture density with sphere of radius 100 nanometer

Geometry, Fields and Transportation
1

I have a sphere of radius 100 nanometers. I am including a material that is a mixture of the following elements with given mass fractions:

{H2O, 5perCent},
{Air, 2perCent},
{H, 3perCent},
{C, 9perCent},
{N, 3perCent},
{O, 2perCent},
{K, 26perCent},
{Na, 18perCent},
{Ca, 15perCent},
{Mg, 10perCent},
{S, 2perCent},
{P, 2perCent},
{Si, 1perCent},
{Al, 1perCent},
{Fe, 1*perCent}

My question is how should I calculate the density of this mixture since I need to specify that value as shown below

G4Material* mixture = new G4Material(name="myMixture ",density=???,ncomponents=14);
Air->AddElement(Air, fractionmass=2*perCent);
Air->AddElement(H, fractionmass=3*perCent);
... goes on