Grains of fine California beach sand are approximately spheres with an average radius of 50μm and are made of silicon dioxide, which has a density of 2.8 × 103 kg/m3. What mass of sand grains would have a total surface area (the total area of all the individual spheres) equal to the surface area of a cube 1.1 m on an edge?

I have calculated the surface areas of the sand but i am stumped from there on... please help.