# Bioprocess Engineering Questions and Answers – Estimating Oxygen Solubility

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Estimating Oxygen Solubility”.

1. The partial pressure of oxygen at 1atm is ________________
a) 0.2000 atm
b) 0.2098 atm
c) 0.2099 atm
d) 0.2096 atm

Explanation: The mole fraction of oxygen in air is 0.2099, so the partial pressure of oxygen at 1 atm air pressure is 0.2099 atm.

2. What is the unit of oxygen solubility “C*AL”?
a) mgl-1
b) mg-1l-1
c) m-1g-1l-1
d) mgl

Explanation: Respiration is expressed in molar units related to biochemical stoichiometries. C*AL is oxygen solubility in units of mg l-1.

3. The addition of ions and sugars added to the fermentation increases the oxygen solubility?
a) True
b) False

Explanation: Oxygen solubility is decreased by the ions and sugars normally added to fermentation media. In a typical fermentation medium, oxygen solubility is between 5% and 25% lower than in water as a result of solute effects. In simple systems, with all the components being dissolved, C0 represents the oxygen solubility and linearly decreases with increasing solute concentrations. In complex solutions with multi-phase structure an increase in C0 can be detected. It suggests that C0 consists of two components — one being oxygen solubility, the other being determined by the amount of oxygen adsorbed on the interphase and bound by macromolecules. The presence of biomass leads to a decrease in C0.

4. The solubility of oxygen in water is temperature and pressure dependent?
a) True
b) False

Explanation: The solubility of oxygen in water is temperature and pressure dependent. About twice as much (14.6 mg•L-1) dissolves at 0 °C than at 20 °C (7.6 mg•L-1), this estimates that at high temperature oxygen solubility is low. Less oxygen dissolves at high elevations compared to low elevations because the atmospheric pressure is less and thus the partial pressure is lower, this estimates that at low pressure the oxygen solubility is also low.

5. “The amount of air dissolved in a fluid is proportional to the pressure in the system”, which law is applicable to this statement?
a) Raoult’s law
b) Fick’s law
c) Henry’s law
d) Newton’s law

Explanation: Solubility of air in water follows Henry’s Law – “the amount of air dissolved in a fluid is proportional to the pressure in the system” – and can be expressed as:
c = pg / kH
where, c = solubility of dissolved gas
kH = proportionality constant depending on the nature of the gas and the solvent
pg = partial pressure of gas (Pa, psi)
The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35.6% oxygen compared to 21% in air.
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6. Henry Law’s Constants at a system temperature of 25°C (77°F) of oxygen is 756.7 atm/(mol/litre). Molar Weight of O2 is 31.9988 g/mol and partial fraction in air is ~ 0.21. Calculate the Oxygen dissolved in the Water at atmospheric pressure.
a) 0.0090 g/liter
b) 0.0089 g/liter
c) 0.0080 g/liter
d) 0.0099 g/liter

Explanation: Oxygen dissolved in the Water at atmospheric pressure can be calculated as:
c = pg / kH
co = (1 atm) 0.21 / (756.7 atm/(mol/litre)) (31.9988 g/mol)
= 0.0089 g/liter.

7. Henry Law’s Constants at a system temperature of 25°C (77°F) of nitrogen is 1600 atm/(mol/litre). Molar weight of N2 is 28.0134 g/mol and partial fraction in air is ~ 0.79. Calculate the Nitrogen dissolved in the Water at atmospheric pressure.
a) 0.0138 g/liter
b) 0.0130 g/liter
c) 0.0132 g/liter
d) 0.0134 g/liter

Explanation: Nitrogen dissolved in the Water at atmospheric pressure can be calculated as:
c = pg / kH
cn = (1 atm) 0.79 / (1600 atm/(mol/litre)) (28.0134 g/mol)
= 0.0138 g/liter.

8. Refer to Q6 and Q7, and calculate the air dissolved in water?
a) 0.0228 g/liter
b) 0.0223 g/liter
c) 0.0227 g/liter
d) 0.0222 g/liter

Explanation: Since air mainly consists of Nitrogen and Oxygen – the air dissolved in the water can be calculated as:
ca = (0.0089 g/litre) + (0.0138 g/litre)
= 0.0227 g/liter.

9. The dissolved oxygen decreases when?
a) The temperature is increased
b) The pressure is increased
c) The salinity is decreased
d) The salinity is increased

Explanation: Dissolved oxygen decreases exponentially as salt levels increase. That is why, at the same pressure and temperature, saltwater holds about 20% less dissolved oxygen than freshwater.

10. The Henry’s law constant for O2 in water at 25°C is 1.27×10−3M/atm and the mole fraction of O2 in the atmosphere is 0.21. Calculate the solubility of O2 in water at 25°C at an atmospheric pressure of 1.00 atm.
Strategy: ▪ Use Dalton’s law of partial pressures to calculate the partial pressure of oxygen.
▪ Use Henry’s law to calculate the solubility, expressed as the concentration of dissolved gas.
a) 2.5×10-4 M
b) 2.1×10-4 M
c) 2.3×10-4 M
d) 2.7×10-4M

Explanation: According to Dalton’s law, the partial pressure of O2 is proportional to the mole fraction of O2:
PA=XAPt=(0.21)(1.00atm)=0.21atm
From Henry’s law, the concentration of dissolved oxygen under these conditions is:
CO2 = KpO2 = (1.27×10−3M/atm) (0.21atm) = 2.7×10-4 M.

11. The value of Henry’s law constant increases with increasing temperature?
a) True
b) False

Explanation: The value of the Henry’s law constant is found to be temperature dependent. The value generally increases with increasing temperature. As a consequence, the solubility of gases generally decreases with increasing temperature. The decrease in solubility of gases with increasing temperature is an example of the operation of Le Chatelier’s Principle. The heat or enthalpy change of the dissolution reaction of most gases is negative, which is to say the reaction is exothermic. As a consequence, increasing the temperature leads to gas evolution.

12. Does yeast need oxygen in fermentation process?
a) True
b) False

Explanation: Oxygen is a critical additive in brewing. Oxygen is the only necessary nutrient not naturally found in wort. Adding adequate oxygen to wort requires a fundamental understanding of why yeast need oxygen, how much oxygen they need, and how to get oxygen into solution and the factors affecting solubility of oxygen. Yeast use oxygen for cell membrane synthesis. Without oxygen, cell growth will be extremely limited. Yeast can only produce sterols and certain unsaturated fatty acids necessary for cell growth in the presence of oxygen.

13. Which type of homebrewer is best for 8 ppm of dissolved oxygen in solution?
a) Siphon sprays
b) Whipping
c) Splashing and shaking
d) Pure air through a stone with an aquarium pump

Explanation: Homebrewers have several aeration/oxygenation methods available to them: siphon sprays, whipping, splashing, shaking, pumping air through a stone with an aquarium pump, and injecting pure oxygen through a sintered stone. Pumping compressed air through a stone is not an efficient way to provide adequate levels of DO. Traditional splashing and shaking, although laborious, is fairly efficient at dissolving up to 8 ppm oxygen. To increase levels of oxygen, the carboy headspace can be purged with pure oxygen prior to shaking.

14. The atmospheric pressure is 1.0 atm and Henry’s law constant for O2 is 1.66 x 10−6 M/mm Hg at 25 °C. Assume air contains 21% oxygen. Calculate the partial pressure of oxygen. (21% of air is oxygen and the mole fraction of O2 is 0.21).
a) 180 mm Hg
b) 130 mm Hg
c) 120 mm Hg
d) 160 mm Hg

Explanation: The partial pressure of oxygen is:
P (O2) = (1.0 atm) ((760 mm Hg)/(1 atm)) (0.21) = 160 mm Hg.

15. Refer to Q14 and, Calculate the solubility of oxygen in units of grams of oxygen per liter of water.
a) 0.0080 g/L
b) 0.0082 g/L
c) 0.0083 g/L
d) 0.0085 g/L

Explanation: The solubility of oxygen in units of grams of oxygen per liter of water is:

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