Asid lemah, penyangga dan keputusan peng titisan
Diperkenalkan| English | Makna |
|---|---|
| dissociation | disosiasi |
| buffer/ˈbʌfə/ | larutan penyangga |
| conjugate base/ˈkɒndʒuːɡeɪt beɪs/ | basa konjugat |
| equivalence/ɪˈkwɪvələns/ | titik ekuivalen |
Which pH model applies?
- A weak acid and a strong acid of the same concentration need not have the same pH. Strength concerns dissociation 解离, not the amount dissolved.
- Identify the acid/base species and any prior neutralisation before choosing a formula.
Use the weak-acid approximation
- For HA, $K_a=[H^+][A^-]/[HA]$. With small dissociation and negligible water contribution, $[H^+]\approx\sqrt{K_ac}$.
- For $K_a=1.80\times10^{-5}\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$ and $c=0.100\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$, $[H^+]\approx0.00134\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$ and pH ≈ 2.87. Dissociation is $(0.00134/0.100)\times100=1.34\%$, supporting the approximation.
Asid kuat monoprotik cair mempunyai [H⁺] = 0.0100 dalam mol per decimeter padu. Cari pH.
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pH = −log₁₀(0.0100) = 2.00.
Buffers 缓冲溶液 react with added reagent
- A buffer contains substantial weak acid HA and conjugate base 共轭碱 A⁻. Added H⁺ consumes A⁻; added OH⁻ consumes HA.
- First react the moles. Adding 0.00400 mol OH⁻ to 0.0100 mol HA leaves 0.00600 mol HA and makes 0.00400 mol A⁻.
At 298 K, $[OH^-]=0.0100\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$ gives $[H^+]=K_w/[OH^-]=10^{-12}\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$, so pH = 12.00 using $K_w=10^{-14}\,\dfrac{\mathrm{mol}^2}{\mathrm{dm}^6}$.
0.0100 mol HA menerima 0.00300 mol OH⁻. Apakah nisbah HA:A⁻ yang tinggal?
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Penutralsian menggunakan 0.00300 mol HA dan membentuk jumlah tersebut A⁻, meninggalkan 0.00700 mol HA.
Komponen tapak konjugat bagi penyangga asid menampung ion _____ yang ditambah.
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A⁻ + H⁺ → HA menampung ion hidrogen yang ditambah.
Use the remaining ratio
- For the buffer, $[H^+]\approx K_a[HA]/[A^-]$. The common final volume cancels when using the remaining mole ratio.
- With $K_a=1.80\times10^{-5}\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$ and HA:A⁻ = 6:4, $[H^+]=2.70\times10^{-5}\,\dfrac{\mathrm{mol}}{\mathrm{dm}^3}$ and pH = 4.57. Both components must remain.
Read the curve before choosing an indicator
- At half-neutralisation of weak acid by strong base, pH = pKa. At equivalence 等当点, the reacting amounts match the balanced equation; pH need not be 7.
- An indicator transition must lie within the steep part of the curve. Beyond equivalence, excess strong reagent controls pH and the acid-buffer formula fails.
“A weak acid is always dilute” is false. Concentration and extent of dissociation describe different properties.
Do stoichiometry before equilibrium. Check whether the proposed formula still has the species it requires.
Setiap titik kesetaraan asid–tapak mesti mempunyai pH 7.
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Kesetaraan asid lemah/basa kuat berada di atas 7 pada 298 K; kesetaraan asid kuat/basa lemah berada di bawah 7.
Apakah dua syarat yang mengesahkan pemilihan penunjuk?
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Titik akhir mesti memberi tindak balas terhadap penambahan kecil berhampiran kesetaraan. pH permulaan tidak menentukan peralihan yang sesuai.