Density = (mass) / (volume)
Volume of your block = (5cm x 5cm x 5cm) = 125 cm³
5 gm/cm³ = (mass) / (125 cm³)
Multiply each side by (125 cm³): Mass = 625 gm
Note:
I don't know what kind of frozen substance you're working with,
but maybe you ought to be careful handling it ... I know that's
not water ice you have there. The density of water ice is not
5 gm/cm³. In fact, it's a little less than 1 gm/cm³. That's why
the ice floats in your soda.