Verifying a Labeled Pure Solid That Is Actually a Mixture
A student receives a 10.0 g sample of white crystals labeled pure and is asked to verify the label by measuring melting point and solubility. The student finds that 6.8 g dissolves in 50 mL of distilled water at 25 °C, leaving 3.2 g of white residue. The filtrate conducts electricity strongly; the residue does not conduct when dry but melts at 80–82 °C. The original solid softens from 76 °C to 84 °C and is completely liquid at 86 °C. A second 10.0 g sample of the original solid produces the same residue mass after washing and drying. The teacher says the sample may contain sodium chloride and an organic compound. The student has available a balance (±0.01 g), hot plate, ice bath, filter paper, conductivity tester, and distilled water. The student claims the broad melting range is due to slow heating and that the sample is a pure compound with a high melting point.
1. Evaluate the student’s claim that the sample is a pure compound. Use at least three independent observations and explain what each indicates.
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The claim is not supported. A pure compound normally melts over a narrow range of about 1–2 °C, but this sample softens from 76 to 84 °C and is liquid only at 86 °C, an 8–10 °C range. Partial solubility is also decisive: 6.8 g dissolves and 3.2 g remains, so the sample separates into at least two fractions. The filtrate conducts strongly, indicating an ionic solute such as NaCl; the dry residue does not conduct and melts at 80–82 °C, consistent with a molecular organic compound. Reproducing the same 3.2 g residue from a second sample shows the composition is fixed, not a heating artifact. Slow heating can broaden a melting range slightly, but it cannot create a conducting filtrate plus a nonconducting residue or a two-component mass split. The sample is a mixture.
2. Design a separation and calculate the mass percent composition. State any assumption and explain how washing the residue affects the result.
Read the worked answer
Add 50 mL distilled water, stir, and filter. Wash the residue with two small portions of cold distilled water, dry it to constant mass, and weigh it as the organic component (3.2 g). Evaporate the filtrate to recover NaCl; if the original 10.0 g is accounted for, NaCl mass is 6.8 g. Mass percent NaCl = (6.8/10.0) × 100 = 68%; organic = 32%. Assumption: no material is lost during transfer and the residue is pure organic compound. Washing is necessary because without it, dissolved NaCl remains in the residue, making the organic mass too high and the NaCl mass too low. Overwashing could dissolve a little organic compound, so use minimal cold water and check constant mass.