Density = mass/volume.

Mass = Density * Volume.

Given

density = 1g/ml

Volume = 2.56 ml

Mass = 1 * 2.56 = 2.56 g.

We know 1 mole contain 6.02 x 10^23 molecules.

Moles = mass/molar mass

= 2.56/18 [ Molar mass of water = 18]

= 0.142.

Therefore 0.142 moles of water will contain 0.142 * 6.02 x 10^23 molecules = 8.5484 * 10^22 molecules.

So 2.56 ml of water at that temperature contain 8.5484 * 10^22 molecules.