How many milliliters of chcl3 are needed to prepare a 2.5% v/v solution in 500 ml of methanol?

You would need 2.5ml CHCl3 in CH3OH to a final solution volume of 100ml Which is: 2.5*5 = 12.5ml CHCl3 in CH3OH to a final solution volume of 500ml. Be careful: mixing liquids very often means that the volumes are not additive: It is therefore not correct to say: add 12.5ml CHCl3 to 487.5ml CH3OH.

The only example among these that is a water-conservation technique that also reduces water pollution is c) factories reusing cooling water instead of returning it to a river.