A boundary tells you which material to count. Concentration adds a denominator.
Label what the scale includes
A sealed container forms a gas during a reaction. If all material stays in the sealed vessel, the scale still supports the gas, liquids and vessel. The appearance of bubbles changes how matter is distributed, not the total mass measured under the stated conditions.
For an open vessel that loses material, compare the same boundary before and after. If the prompt says gas was the only material that crossed it, the decrease estimates gas mass. If it also mentions a spill or evaporation, separate those pathways before assigning the whole change to a single cause.
Separate total solute and concentration
A 0.20 L portion of a 30 g/L solution contains 6 g of solute. Adding water until the solution reaches 0.50 L leaves 6 g, assuming no solute is lost. Now use 6 divided by 0.50 to get 12 g/L. The dilution changed the amount per liter without changing the amount in this portion.
If you compare different vessels, align units and decide whether the question is about grams or grams per liter. The vessel with more total solute can have lower concentration if its volume is sufficiently larger.
Apply the guide
An open vessel drops from 93 g to 89 g. Gas is the only material crossing the boundary. How much gas escaped?Reveal explanation
4 g, the initial measured mass minus the final measured mass.
A solution has 8 g of solute in 0.25 L. Water is added until the volume is 0.80 L. What is its final concentration?Reveal explanation
10 g/L. The solute remains 8 g, so divide 8 by 0.80.
Turn the idea into practice.
Apply the rule to a new case, then explain your choice.
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Original OpenExam guide
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OpenStax Chemistry 2e 3.3 MolarityOpenStax Chemistry 2e 1.2 Phases and Classification of Matter