Hey there!
The density of water is 1 g/mL.
That means 250 mL of water is 250 grams of water.
Let's find the molar mass:
H: 2 x 1.008 = 2.016
O: 1 x 16 = 16
-----------------------------
18.016 g/mol
Find the number of moles we have.
250 ÷ 18.016 = 13.88
One mole is 6.022 x 10²³ molecules and we have 13.88 moles.
13.88 x 6.022 x 10²³ = 8.4 x 10²⁴
There are 8.4 x 10²⁴ molecules in 250 mL of water.
Hope this helps!