A 0.150 L sample of 0.556 M NaCl is left to evaporate until only 0.105 L remains. What is the new concentration?

1. Evaporation removes solvent only, so the amount of NaCl in the flask never changes. 2. That amount is 0.556 mol/L x 0.150 L = 0.0834 mol of NaCl. 3. The same moles now sit in a smaller volume of 0.105 L. 4. The new concentration is 0.0834 mol / 0.105 L = 0.794 mol per litre. 5. Losing volume with fixed solute must raise the concentration, so any answer below 0.556 M fails the sanity check. 6. The value 0.389 M applies the volume ratio the wrong way round, as though the solution had been diluted. 7. The value 0.556 M assumes concentration cannot change, and 0.0834 M reports the mole count as a molarity. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.3 Molarity_