Question

how much water is to be added to prepare 0.12M solution from 100ml of 0.3M solution​

Answer 1

Explanation:

You can easily answer by understanding the definition.

A one M (one Molar) solution has one mole per liter of solution, your starting solution is one liter at 0.5M, you then have 0,5 moles of NaCl total. To get it down to .2moles per liter, you have to dilute 2.5 times (0.5/0.2=2.5) So the total volume should be 2.5 liters, starting with one liter, you need to add 1.5 liters of water.

The problem is not really well asked since there is no way to make only one liter of 0.2M starting with a 0.5M and add water. Diluting the starting 0.5M solution, will get the 0.2M but 2.5 liters of it.