Higher Tier, Chemistry-only: concentration in moles per solution volume
הוצג| English | משמעות |
|---|---|
| molar concentration/ˈməʊlə ˌkɒnsənˈtreɪʃn/ | ריכוז מולרי |
| aliquot/ˈælɪkwɒt/ | אליקוואט |
What would explain this observation?
- Higher Tier: A concentration of 0.20 moles per cubic decimetre tells the amount of dissolved solute, not its gram mass. The molar mass is needed to connect amount and mass.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Molar concentration 物质的量浓度 c=n/V uses solute amount n in mol and final solution volume V in dm³. Rearrange to n=cV or V=n/c. To find solute mass use m=nM, where M is molar mass in grams per mol. Combining gives c=m/(MV). The same gram mass of two different solutes can have different mole amounts because their molar masses differ.
- molar concentration: The amount in moles of stated solute per volume of solution; aliquot 定量分取液: A measured portion of a larger sample of solution.
Which sequence finds solute mass from molar concentration?
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Concentration in mol per dm^3 needs the amount in moles first: n = cV, then convert to mass with m = nM. Skipping the molar mass leaves grams out of reach, and cm^3 must become dm^3 before use.
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
- Convert cm³ to dm³ by dividing by 1,000. A 250 cm³ portion of 0.20 $\dfrac{\text{mol}}{\text{dm}^3}$ solution contains 0.20×0.250=0.050 mol solute. If solute molar mass is 40 $\dfrac{\text{g}}{\text{mol}}$, that amount weighs 2.0 g. At fixed amount, greater final solution volume means lower molar concentration. At fixed final volume, more solute moles means higher concentration.
- Write units beside concentration and volume before multiplying. Identify which chemical species concentration describes. A bottle’s molar concentration normally refers to the stated solute formula amounts, not automatically the total amount of every ion after dissociation. Actual solution preparation requires a measured final volume, complete dissolution and the teacher-approved apparatus and procedure.
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: 4.0 g NaOH with supplied molar mass 40 $\dfrac{\text{g}}{\text{mol}}$ is 0.100 mol. In final volume 0.500 dm³ its concentration is 0.100/0.500=0.200 $\dfrac{\text{mol}}{\text{dm}^3}$. A 100 cm³ aliquot contains 0.0200 mol and 0.800 g NaOH. This is solute content rather than total solution mass.
A 200 cm³ solution contains 0.30 $\dfrac{\text{mol}}{\text{dm}^3}$ solute. Calculate its amount. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A 200 cm³ solution contains 0.30 mol per dm³ solute. Calculate its amount.
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The result is 0.06 mol. Known: V = 200 cm^3 = 0.200 dm^3; c = 0.30 mol per dm^3. Use n = cV. n = 0.30 x 0.200 = 0.06 mol. Mass would need one more step with the molar mass.
Check the conclusion and its limits
- Do not substitute grams directly into c=n/V. Do not use the volume of solvent added when a final solution volume is specified. Molar concentration and mass concentration are related through molar mass, but their numerical values and units are different.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A molar concentration states grams of solute per dm³. This claim is false: Molar concentration counts MOLES per dm^3. The same solution also has a mass concentration in grams per dm^3, and the two differ by the molar mass - 0.1 mol per dm^3 of NaOH is 4 g per dm^3, not 0.1 g.
Higher Tier, Chemistry-only: concentration in moles per solution volume: Convert cm³ to dm³ by dividing by 1,000. A 250 cm³ portion of 0.20 $\dfrac{\text{mol}}{\text{dm}^3}$ solution contains 0.20×0.250=0.050 mol solute. If solute molar mass is 40 $\dfrac{\text{g}}{\text{mol}}$, that amount weighs 2.0 g. At fixed amount, greater final solution volume means lower molar concentration. At fixed final volume, more solute moles means higher concentration.
A molar concentration states grams of solute per dm³.
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Molar concentration counts MOLES per dm^3. The same solution also has a mass concentration in grams per dm^3, and the two differ by the molar mass - 0.1 mol per dm^3 of NaOH is 4 g per dm^3, not 0.1 g.
The amount in moles of stated solute per volume of solution: write the technical term.
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molar concentration means The amount in moles of stated solute per volume of solution.