An empty 1000 mL container has a mass of 250 g when filled with a liquid that container in the liquid have a combined mass of 1300 g

Total Weight - Container weight = Liquid Weight

1300 - 250 = 1050 / 1.05 = 1000 (amount filled in the container)

So

1050 g

The weight of each g/ml must be 1.05 in order to get 1050 ml.

The answer is 1.05

(Credit given to user "Aakasha A.', information above used only to pass on information that has been answered already)