Question

Robert says that when 1.00 L of a NaCl solution is diluted to 1.80 L, the concentration of the solution decreases.
But Matthew says that the number of moles of NaCl stays the same between the two solutions.
Who is correct?

Answer 1

Concentration = moles of solute present in one liter of solution.

Concentration = moles / 1L solution

As the solution is diluted then there is no change in the number of moles of solute present the the solution but volume of solution increases. This indicates that relative quantity of solute with respect to solution is less. So, concentration of solution is decreases.

1.0 L solution and 1.8 L solution, both will contains same number of moles of NaCl.

With respect to concetration terms there is difference but with respect to number of moles of solute is same.

Both are correct in the view of their expression as one with concentration and other with number of moles of solute.