500 mL of a 2.5 M solution of NaCl is left uncovered and 200 mL of the solvent evaporated out. What is the new molarity of the solution?

Answer 1

500 mL of a 2.5 M solution contains 2.5 M/1000mL x 500 mL = 1.25 moles. After 200 mL evaporated, there is only 300 mL left, so the molarity would be 1.25 moles / 300 mL x 1000 mL = 4.2 M

Remember that 2.5 M means 2.5 moles per liter which is equivalent to 2.5 moles per 1000 mL.