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Chemical Engineering Principles — Practice Q&A
1. A solution contains 40.0 g of NaCl dissolved in 160.0 g of
water. What is the mass fraction and mass percentage of NaCl?
Answer: Mass fraction = 0.20; mass % = 20.0%
Solution:
Total mass = 40.0 + 160.0 = 200.0 g
Mass fraction NaCl = 40..0 = 0.20
Mass % = 0.20 × 100 = 20.0%
2. A gas mixture contains 3.0 mol N₂ and 1.0 mol O₂. What is
the mole fraction of oxygen?
Answer: 0.25
Solution:
Total moles = 3.0 + 1.0 = 4.0 mol
Mole fraction O₂ = 1..0 = 0.25
3. Convert a pressure of 2.50 atm to kPa.
Answer: 253 kPa
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Solution:
1 atm = 101.325 kPa
P = 2.50 × 101.325 = 253.3 kPa ≈ 253 kPa
4. Convert a temperature of 68°F to °C and K.
Answer: 20°C; 293.15 K
Solution:
°C = (68 − 32) × 5/9 = 20°C
K = 20 + 273.15 = 293.15 K
5. A gas occupies 10.0 L at 2.00 atm and 25°C. How many moles
of gas are present?
Answer: 0.818 mol
Solution:
Ideal gas law: PV = nRT
T = 25 + 273.15 = 298.15 K
n = PV / RT = (2.00 atm × 10.0 L) / (0.08206 L·atm/mol·K ×
298.15 K)
n ≈ 0.818 mol
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6. A fuel gas contains 85 mol% methane (CH₄) and 15 mol%
ethane (C₂H₆). What is the average molecular weight of the
mixture?
Answer: 18.1 g/mol
Solution:
MW CH₄ = 16.04; MW C₂H₆ = 30.07
Average MW = 0.85(16.04) + 0.15(30.07) = 13.63 + 4.51 = 18.14
g/mol
7. A stream has a mass flow rate of 100 kg/min and contains 30
wt% ethanol and 70 wt% water. What is the mass flow rate of
ethanol?
Answer: 30 kg/min
Solution:
Ethanol flow = 0.30 × 100 = 30 kg/min
8. A distillation column is fed 100 mol/h of a mixture containing
40 mol% benzene (B) and 60 mol% toluene (T). The distillate is
90 mol% benzene. If the distillate flow rate is 30 mol/h, what is
the benzene recovery in the distillate?
Answer: 67.5%
Chemical Engineering Principles — Practice Q&A
1. A solution contains 40.0 g of NaCl dissolved in 160.0 g of
water. What is the mass fraction and mass percentage of NaCl?
Answer: Mass fraction = 0.20; mass % = 20.0%
Solution:
Total mass = 40.0 + 160.0 = 200.0 g
Mass fraction NaCl = 40..0 = 0.20
Mass % = 0.20 × 100 = 20.0%
2. A gas mixture contains 3.0 mol N₂ and 1.0 mol O₂. What is
the mole fraction of oxygen?
Answer: 0.25
Solution:
Total moles = 3.0 + 1.0 = 4.0 mol
Mole fraction O₂ = 1..0 = 0.25
3. Convert a pressure of 2.50 atm to kPa.
Answer: 253 kPa
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Solution:
1 atm = 101.325 kPa
P = 2.50 × 101.325 = 253.3 kPa ≈ 253 kPa
4. Convert a temperature of 68°F to °C and K.
Answer: 20°C; 293.15 K
Solution:
°C = (68 − 32) × 5/9 = 20°C
K = 20 + 273.15 = 293.15 K
5. A gas occupies 10.0 L at 2.00 atm and 25°C. How many moles
of gas are present?
Answer: 0.818 mol
Solution:
Ideal gas law: PV = nRT
T = 25 + 273.15 = 298.15 K
n = PV / RT = (2.00 atm × 10.0 L) / (0.08206 L·atm/mol·K ×
298.15 K)
n ≈ 0.818 mol
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6. A fuel gas contains 85 mol% methane (CH₄) and 15 mol%
ethane (C₂H₆). What is the average molecular weight of the
mixture?
Answer: 18.1 g/mol
Solution:
MW CH₄ = 16.04; MW C₂H₆ = 30.07
Average MW = 0.85(16.04) + 0.15(30.07) = 13.63 + 4.51 = 18.14
g/mol
7. A stream has a mass flow rate of 100 kg/min and contains 30
wt% ethanol and 70 wt% water. What is the mass flow rate of
ethanol?
Answer: 30 kg/min
Solution:
Ethanol flow = 0.30 × 100 = 30 kg/min
8. A distillation column is fed 100 mol/h of a mixture containing
40 mol% benzene (B) and 60 mol% toluene (T). The distillate is
90 mol% benzene. If the distillate flow rate is 30 mol/h, what is
the benzene recovery in the distillate?
Answer: 67.5%
