Higher Tier, Chemistry-only: determine acid concentration from RP2 data
| English | 中文 | Pinyin |
|---|---|---|
| known concentration | 已知浓度 | yǐ zhī nóng dù |
| titration concentration | 滴定求得浓度 | dī dìng qiú dé nóng dù |
What would explain this observation?
- Higher Tier: A known sodium hydroxide solution can determine an unknown sulfuric-acid concentration. The acid amount is half the alkali amount because the equation requires two NaOH per H₂SO₄.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- For 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, n(acid)=n(NaOH)/2. First calculate alkali amount as concentration times pipetted volume in dm³. Divide by two, then divide acid amount by mean acid titre in dm³. Convert acid molar concentration to mass concentration by multiplying by its molar mass. For supplied H=1, S=32 and O=16, H₂SO₄ has molar mass 98 $\dfrac{\text{g}}{\text{mol}}$.
- known concentration 已知浓度: The supplied concentration of a reference solution used in the calculation; titration concentration 滴定求得浓度: A concentration inferred from reacting amounts and measured solution volumes.
For 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, how do acid and alkali amounts relate?
The acquired handbook PDF page 35 correctly states the equation and the 2:1 ratio, but prints a reversed half-mole statement afterwards. The correct inference is acid moles = half alkali moles. The balanced equation gives the correct amount relationship. The common-tier RP2 task still concerns accurate reacting volumes, while this numerical determination is Higher-only.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- The acquired handbook PDF page 35 correctly states the equation and the 2:1 ratio, but prints a reversed half-mole statement afterwards. The correct inference is acid moles = half alkali moles. The balanced equation gives the correct amount relationship. The common-tier RP2 task still concerns accurate reacting volumes, while this numerical determination is Higher-only.
- Use the measured accurate mean from the student’s actual supervised task, including the stated trial selection. Record known solution concentration and volume, convert cm³ to dm³, and label each chemical’s amount. If the question supplies different acid identity or coefficient ratio, change the calculation accordingly. A calculated concentration is not a direct burette reading.
Which two habits make the investigation or model in this case more defensible?
Use the measured accurate mean from the student’s actual supervised task, including the stated trial selection. Record known solution concentration and volume, convert cm³ to dm³, and label each chemical’s amount. If the question supplies different acid identity or coefficient ratio, change the calculation accordingly. A calculated concentration is not a direct burette reading.
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: 25.0 cm³ NaOH at 0.100 $\dfrac{\text{mol}}{\text{dm}^3}$ contains 0.00250 mol. Acid amount=0.00125 mol. With acid mean titre 12.50 cm³, acid concentration=0.00125/0.01250=0.100 $\dfrac{\text{mol}}{\text{dm}^3}$. Mass concentration=0.100×98=9.80 $\dfrac{\text{g}}{\text{dm}^3}$. Both units describe the same solution using different quantities.
An inferred H₂SO₄ concentration is 0.150 $\dfrac{\text{mol}}{\text{dm}^3}$. With molar mass 98 $\dfrac{\text{g}}{\text{mol}}$, find its mass concentration. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
An inferred H₂SO₄ concentration is 0.150 mol per dm³. With molar mass 98 g per mol, find its mass concentration.
The result is 14.7 g per dm³. Known: 25.0 cm³ NaOH at 0.100 mol per dm³ contains 0.00250 mol. Acid amount=0.00125 mol. With acid mean titre 12.50 cm³, acid concentration=0.00125/0.01250=0.100 mol per dm³. Mass concentration=0.100×98=9.80 grams per dm³. Both units describe the same solution using different quantities.
Check the conclusion and its limits
- Do not multiply the alkali amount by two for this acid. Do not use centimetre-cubed volumes without conversion when concentration uses dm³. The result assumes the stated pure-solution reaction, correct endpoint and sufficiently accurate measurements.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
The handbook’s printed half-mole statement overrides its correctly balanced equation. This claim is false: Do not multiply the alkali amount by two for this acid. Do not use centimetre-cubed volumes without conversion when concentration uses dm³. The result assumes the stated pure-solution reaction, correct endpoint and sufficiently accurate measurements.
Higher Tier, Chemistry-only: determine acid concentration from RP2 data: The acquired handbook PDF page 35 correctly states the equation and the 2:1 ratio, but prints a reversed half-mole statement afterwards. The correct inference is acid moles = half alkali moles. The balanced equation gives the correct amount relationship. The common-tier RP2 task still concerns accurate reacting volumes, while this numerical determination is Higher-only.
The handbook’s printed half-mole statement overrides its correctly balanced equation.
Do not multiply the alkali amount by two for this acid. Do not use centimetre-cubed volumes without conversion when concentration uses dm³. The result assumes the stated pure-solution reaction, correct endpoint and sufficiently accurate measurements.
The supplied concentration of a reference solution used in the calculation: write the technical term.
known concentration means The supplied concentration of a reference solution used in the calculation.