2 free 4A A-2 questions, explained
Original exam-style questions written to the public syllabus, from our free public sample. They are not questions from a real exam.
1. At room temperature (approximately 20°C) and atmospheric pressure, which of the following lists the phases of steam, nitrogen, copper, lemonade, and cast iron correctly?
- A. Gas, liquid, solid, solid, solid
- B. Gas, gas, solid, liquid, solid
- C. Gas, gas, liquid, liquid, solid
- D. Liquid, gas, solid, liquid, solid
Show the answer and explanation
Answer: B. Steam is water in the gaseous phase; nitrogen (N2, boiling point −196°C) is a gas at room temperature; copper (a metal) and cast iron (an iron alloy) are both solids; lemonade is an aqueous liquid mixture. The correct sequence is gas, gas, solid, liquid, solid. Choices that list lemonade as a solid or nitrogen as a liquid are incorrect under standard room conditions.
Question ID KE-A02-C01-Q001 · Unit A-2: Elementary Physical, Chemical, and Thermodynamic Principles · Report an error
2. An industrial steam coil heats 1500 kg/h of water from 30°C to 70°C. The coil is supplied with 95% dry steam at 1250 kPa; condensate leaves at 80°C. Using steam table values at 1250 kPa (hf ≈ 806 kJ/kg; hfg ≈ 1978 kJ/kg) and hf at 30°C ≈ 125.8 kJ/kg, hf at 70°C ≈ 293.0 kJ/kg, hf at 80°C ≈ 334.9 kJ/kg, and assuming no heat losses, which answer is closest to the required steam consumption per hour?
- A. Approximately 130 kg/h
- B. Approximately 80 kg/h
- C. Approximately 107 kg/h
- D. Approximately 165 kg/h
Show the answer and explanation
Answer: C. Heat needed by the water per hour = 1500 × (293.0 − 125.8) = 1500 × 167.2 = 250 800 kJ/h. Enthalpy of the 95% dry supply steam = hf + 0.95 × hfg = 806 + 0.95 × 1978 = 806 + 1879.1 ≈ 2685 kJ/kg. Heat released per kg of steam = 2685.1 − 334.9 = 2350.2 kJ/kg (the condensate leaves at 80°C, so its enthalpy is subtracted). Steam consumption = 250 800 ÷ 2350.2 ≈ 107 kg/h. Using the saturated-steam enthalpy (hg at 1250 kPa ≈ 2784 kJ/kg) instead of accounting for the 95% dryness would understate the steam required, a common error.
Question ID KE-A02-C04-Q007 · Unit A-2: Elementary Physical, Chemical, and Thermodynamic Principles · Report an error
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