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"problem_id": "Exam-05-T4",
"header": "Problem 4 Phosphoric Acid and Phosphate Chemistry. Phosphates are important in industrial production, food additives, and other fields, so measuring their content and understanding their properties are significant.",
"header_images": [],
"subquestions": [
{
"sub_id": "Exam-05-T4-1",
"tags": [
"2.1"
],
"text_open": "4-1 Aqueous sodium phosphate dodecahydrate, Na_3PO_4·12H_2O, is strongly basic and is used in washing powders and fat-saponification agents. It is prepared by reacting calcium dihydrogen phosphate with sodium carbonate and then adding sodium hydroxide solution. Write the preparation equation.",
"text_images": [],
"full_mark": 2,
"rubric": [
"Ca(H_2PO_4)_2+Na_2CO_3 + 4NaOH + 20H_2O = 2[Na_3PO_4·12H_2O] + CaCO_3. (2 points)"
],
"rubric_images": []
},
{
"sub_id": "Exam-05-T4-2",
"tags": [
"4.5"
],
"text_open": "Titration notation: Add a volume $V_0$ of standard HCl of concentration $c_1$ to the sample, then titrate with standard NaOH of concentration $c_2$. The first jump near pH 4.0 consumes a NaOH volume $V_1$, and continuing to the second jump near pH 8.8 consumes an additional volume $V_2$.\n4-2-2 What relation among V_0, V_1, V_2, c_1, and c_2 indicates that the sample contains Na_2HPO_4? Express it symbolically.",
"text_images": [],
"full_mark": 2,
"rubric": [
"$V_0c_1-V_1c_2<2V_2c_2$, or equivalently $V_0c_1<c_2(V_1+2V_2)$. (2 points)"
],
"rubric_images": []
},
{
"sub_id": "Exam-05-T4-3",
"tags": [
"4.5",
"4.7"
],
"text_open": "4-2-3 Calculate the mass fraction ω(%) of Na_3PO_4 in the sample. Significant figures are required.",
"text_images": [],
"full_mark": 4,
"rubric": [
"First determine that $30 × 1.057 - 9.60 × 1.032 < 2 × 10.75 × 1.032$, showing that the sample contains Na_2HPO_4. (2 points)",
"$\\omega =\\frac{30.00\\times 1.057-9.60\\times 1.032-10.75\\times 1.032\\times {{10}^{-3}}\\times 163.94}{2.013}=87.22\\%$. (2 points; directly obtaining n(Na_3PO_4) = 0.01071 mol without the determination also earns 2 points. Deduct 1 point for incorrect significant figures.)"
],
"rubric_images": []
},
{
"sub_id": "Exam-05-T4-4",
"tags": [
"1.7",
"2.1"
],
"text_open": "4-3-2 Phosphorus atoms in different oxidation states can also be linked in catenated phosphoric acids. H_4P_2O_5 contains linked P(Ⅱ) and P(Ⅳ). It has two hydrolysis pathways under basic conditions. Only the first generates gas, and each gives only one phosphorus-containing product. Write the ionic equations for both pathways.",
"text_images": [],
"full_mark": 4,
"rubric": [
"Pathway one: (HP_2O_5)^(3-) + OH^- = (P_2O_6)^(4-) + H_2. (2 points)",
"Pathway 2: (HP_2O_5)^(3-) + OH^- = 2(HPO_3)^(2-). (2 points)",
"Writing reactants or products in acid form earns 1 point per equation. Answers containing (P_2O_5)^(4-) or (PO_3)^(3-) receive no credit."
],
"rubric_images": []
},
{
"sub_id": "Exam-05-T4-5",
"tags": [
"2.4"
],
"text_open": "4-3-3 Hydrolysis of cyclic H_6P_6O_12 is complex. First, one P-P bond cleaves to form a linear catenated hexaphosphoric acid by disproportionation; then terminal P-P bonds cleave stepwise. How many ways can the linear hexaphosphoric acid hydrolyze to catenated pentaphosphoric acid? Give the products of each. Use P for a phosphorus-containing group and a superscript to denote oxidation state; for example, catenated diphosphoric acid is P^4-P^4.",
"text_images": [],
"full_mark": 4,
"rubric": [
"Three. (1 point)",
"P^1 + P^4-P^3-P^3-P^3-P^4. (1 point) P^3 + P^2-P^3-P^3-P^3-P^4. (1 point) P^5 + P^2-P^3-P^3-P^3-P^2. (1 point)",
"Deduct 1 point for an incorrect hydrolysis pathway and 1 point if the monophosphorus product is omitted."
],
"rubric_images": []
}
]
}
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