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.
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