**1.**Dry air can be modeled as aa mixture of 78.084% N2, 20.947% O2, 0.934% Air and 0.035% CO2 (all given on a volume basis).Calculate the mixture molecular weight,and the mass and molar fractions of the four species.At a temperature of 25ºC and a pressure of 101 kPa,what is the molar concentration and density of the mixture? Compare the density with a value from a textbook, and give the reference for the textbook.Note that Wikipedia is not a textbook.

**2.**A pool of water has a temperature of 15ºC. What is the relative humidity of the air/vapor mixture immediately above the surface of the water,and what are the mass and molar fractions of the water vapor? What is the density of the mixture,and how does it compare to dry air at the same temperature? Sketch the flow that would occur due to natural convection if the pool was overlain with dry air at the same temperature.Assume the air is at 15ºC and 1 atm pressure.

**3.**A steel sphere (30 cm internal radius, 1 cm thick wall) contains pure H2 at 10 MPa. What is the moral and mass flow rate of Hydrogen through the wall if

**a)** The external pressure is 100 kPa and external gas is dry air (0% H2).

**b)**The external pressure is 100 kPa and the external gas is a mixture of 90% dry air and 10% H2(percentages given by volume).

Note:you should use the data for Hydrogen-iron diffusivity given in Table 3,and the solubility relationship. where *P *is the gas partial pressure in bar,and *T *is the temperature in Kelvin. Assume a temperature of 50ºC.Hint; *D12 *is on the order of

**4.**A student runs a dilute solution of methylene blue in water through a photo catalytic reactor. The reactor is run on continuous recycle (ie: the fluid is repeatedly passed through thr reactor), and the concentration of the mixture is measured every 15 minutes. The total volume of the solution is 2 L, and the methylene blue has an initial concentration of 7 mg/L.

t (minutes) | Expt 1 C(mg/L) | Expt 2 C(mg/L) | Expt 3 C(mg/L) |

0 | 7.00 | 7.00 | 7.00 |

15 | 4.63 | 2.30 | 2.30 |

30 | 3.06 | 1.53 | 1.53 |

45 | 2.02 | 1.01 | 1.01 |

60 | 1.34 | 0.67 | 0.67 |

Decrease in concentration of methylene blue with time.

The Schmidt number is Sc=750, and the reactor has a surface area of 3.22 × 10-² m2

**a)**Calculate the reaction constant *k**“* for expt.1 assuming that the reaction is rate controlled.

**b)** The experiment is return with the reaction rate doubled (expt. 2) and quadrupled (expt. 3). From the data,what can you say about the reaction for these two experiments? Calculate the equivalent stagnant film thickness for the two experiment.

**c)** Using the equivalent stagnant film thickness calculated in part b), recalculate the reaction constant *k” *from expt.1. Is the original assumption that is rate controlled valid?

Hint: the reaction constant and diffusion thickness are of the order

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