Solution concentration by mass: convert volume before calculating
הוצג| English | משמעות |
|---|---|
| solute/ˈsɒljuːt/ | חומר מומס |
| mass concentration/mæs ˌkɒnsənˈtreɪʃn/ | ריכוז מסתי |
What would explain this observation?
- A bottle labelled 8 grams per cubic decimetre contains 8 g solute 溶质 in each 1 dm³ of solution. A quarter of that solution volume contains a quarter of the solute mass.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Mass concentration 质量浓度 states the mass of dissolved solute in a given volume of solution. With concentration c in $\dfrac{\text{g}}{\text{dm}^3}$, mass m in grams and solution volume V in dm³, m=cV. One dm³ equals 1,000 cm³. Thus 250 cm³ equals 0.250 dm³, not 250 dm³. Use final solution volume rather than assuming the solvent volume is identical.
- solute: The substance dissolved in a solvent to form a solution; mass concentration: The mass of solute per stated volume of solution.
Which volume should be used with a concentration in grams per dm³?
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The concentration is per CUBIC DECIMETRE, so the volume must be in dm^3: 250 cm^3 = 0.250 dm^3. Using 250 with a per-dm^3 concentration is a thousandfold error, and a mass is never a volume.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For a uniformly mixed solution, the amount of solute taken is proportional to the sample volume. A 0.250 dm³ sample of an 8.0 $\dfrac{\text{g}}{\text{dm}^3}$ solution contains 2.0 g solute. The unit calculation cancels dm³ and leaves grams. This shared-tier numerical task is distinct from the embedded Higher-only explanation of how changing mass or solution volume changes concentration.
- Write the concentration unit beside the value and convert volume before multiplying. Mark whether a number refers to solute, solvent or solution. For an actual teacher-approved preparation, weigh the selected solute, dissolve it and make the solution up to the required final volume using appropriate school apparatus. Check complete dissolution and safe handling; the written calculation does not replace actual supervised technique.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: 30 $\dfrac{\text{g}}{\text{dm}^3}$ solution is sampled at 150 cm³. Volume=150/1,000=0.150 dm³, so solute mass=30×0.150=4.5 g. A 300 cm³ portion contains 9.0 g under the same uniform-concentration condition. The sample’s total solution mass is not given by this solute calculation.
Find solute mass in 200 cm³ of a 15 $\dfrac{\text{g}}{\text{dm}^3}$ solution. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Find solute mass in 200 cm³ of a 15 grams per dm³ solution.
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The result is 3 g. Known: 200 cm^3 = 0.200 dm^3; concentration 15 g per dm^3. Use mass = concentration x volume. Mass = 15 x 0.200 = 3 g of solute. This is the solute mass, not the mass of the whole solution.
Check the conclusion and its limits
- Do not multiply concentration by a cm³ number when the concentration uses dm³. Dissolved solute is still present even if invisible. A concentration is not the total mass of the bottle, and equal solution volumes need not contain equal solute masses when concentrations differ.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Solute mass calculated from concentration is the total mass of the solution. This claim is false: Concentration x volume gives the mass of SOLUTE only. The solution also contains the solvent - roughly a kilogram of water per litre - so total solution mass is far larger.
Solution concentration by mass: convert volume before calculating: For a uniformly mixed solution, the amount of solute taken is proportional to the sample volume. A 0.250 dm³ sample of an 8.0 $\dfrac{\text{g}}{\text{dm}^3}$ solution contains 2.0 g solute. The unit calculation cancels dm³ and leaves grams. This shared-tier numerical task is distinct from the embedded Higher-only explanation of how changing mass or solution volume changes concentration.
Solute mass calculated from concentration is the total mass of the solution.
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Concentration x volume gives the mass of SOLUTE only. The solution also contains the solvent - roughly a kilogram of water per litre - so total solution mass is far larger.
The substance dissolved in a solvent to form a solution: write the technical term.
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solute means The substance dissolved in a solvent to form a solution.