1. Understand Molarity

Molarity (M) is a measure of concentration, defined as moles of solute per liter of solution:

M = moles of solute / liters of solution

2. Rearrange the Equation

We want to find the volume (in liters) of solution, so let's rearrange the equation:

liters of solution = moles of solute / M

3. Plug in the Values

* moles of solute = 0.100 mol NaCl

* M = 0.250 M NaCl

liters of solution = 0.100 mol / 0.250 M = 0.400 L

4. Convert to Milliliters

Since 1 L = 1000 mL, we can convert liters to milliliters:

0.400 L * (1000 mL / 1 L) = 400 mL

Answer: You need to measure 400 milliliters of the 0.250 M NaCl solution to obtain 0.100 mol NaCl.