| { |
| "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": [] |
| } |
| ] |
| } |
|
|
