How heavy is 2.5 cubic meters?

It all depends what's in those cubic metres.

If they're full of air and not surrounded by more air, then they weigh about 7.1 pounds.

If they're full of water, then they weigh about 5,512 pounds.

If they're full of stones or lead, then they weigh even more than that.

If they're completely empty, then their weight is zero.