Practice Exams ChE - 13

1. A solution of 30% by weight Na2CO3 in water is cooled to 20 degC. 10% of the water in the solution is evaporated. If the solubility of Na2CO3 is 21.5 grams for 100 grams of H2O, how much Na2CO3•10H2O crystals in grams can be crystallized out of the solution for every kilogram of original solution?

2. Air is passed through a membrane to separate O2 from N2. The final product is found to contain 15% O2 and 85% N2. Find the percentage of O2 in the remaining air if only 30% of air passed through the membrane.
a. 51%
b. 15%
c. ???
d. 26.3%

3. A solution containing equimolal O2 and N2 is passed though a membrane to separate O2 from N2. The final product is found to contain 45% O2 and 55% N2. Find the percentage of O2 remaining if only 205 of the original solution passed through the membrane.
a. 51%
b. 15%
c. 50%
d. 45%

4. The equipment that uses the principle that water cools when some of it is evaporated.
a. evaporator
b. humidifier
c.crystallizer
d. dryer

5. The lowermost layer of a landfill
a. ???
b. impermeable clay
c. ???
d. ???

6. Prandtl number is
a. force/area
b. work/momentum
c. dimensionless
d. ???

7. The mole fraction of a substance in the liquid phase is 0.20. If its vapor pressure is one-half the total pressure, what is its mole fraction in the vapor phase?
a. 0.10
b. 0.50
c. 0.20
d. 0.70

8. How many per hour of H2O flows through a 3-inch Sch. 40 steel pipe with an average velocity of 8cm/sec?
a. 17,150
b. 21,500
c. 36,000
d. 72,300

9. Water at a rate of 0.80 kg/sec is heated from 35?C to 45?C in a 2.5 cm diameter tube whose surface is maintained at 90?C. The required length of tube, in meters, to accomplish the heating is:
a. 3.6
b. 1.0
c. 2.1
d. 2.7

10. The vapor and liquid in the separator for flash distillation are always
a. in equilibrium
b. replenished
c. of equal composition
d. below the boiling point of the solution

11. Small ponds, less than 1 meter deep, whose oxygen supply primarily comes from photosynthetic activity
a. facultative ponds
b. anaerobic ponds
c. aerated lagoons
d. aerobic ponds