This is an exercise taken from Welty's book, but I didn't understand the mass balance used in the solution.

Please explain to me step by step.

A wetted-wall column of 2.0-cm inner diameter and 50-cm wetted length is used to oxygenate blood as a continuous, steady-state process, as shown in the figure below. Blood containing 1.0 gmole/m³ of dissolved oxygen enters the top of the wetted-wall column at a volumetric flow rate of 300 cm³/min. Pure, 100% O₂ gas at 1.0 atm and 25C enters the bottom of the column at a volumetric gas flow rate of 600 cm³/min. A very simplified description for estimating the equilibrium solubility of O₂ dissolved in blood is

where p_A is the partial pressure of O₂ in the gas phase, H =0.8 atm m³/gmole for O₂ in the blood plasma, k= 28 atm^-1 for the blood hemoglobin, and c_AL,max =9.3 gmole O^₂/m³ for the hemoglobin.

At 25C, the kinematic viscosity of blood is 0.040 cm²/s and the density of blood is 1.025 g/cm³. You may assume that the diffusion coefficient of O₂ in blood approximates the diffusion coefficient of O₂ in liquid water, which is 2.0 x10^-5 cm²/s at 25C.

a. What is the mass-transfer coefficient for O₂ into the flowing liquid film?

b. What is the concentration of dissolved oxygen in the liquid exiting the bottom of the column, c_AL,out?

cAL=HpA+1+kpAkpAcAL,max Liquid in (blood + dissolved O2 )