Chapter 5, Problem 5.91P

In a metered-dose inhaler (MDI), such as those used for asthma medication, medicine is delivered by a compressed-gas propellant. (The device is similar in concept to a can of spray paint.) When the inhaler is activated, a ?xed amount of the medicine suspended in the propellant is expelled from the mouthpiece and inhaled. In the past, chloro?uorocarbons (CFCs) were used as propellants; however, because of their reactivity with the Earth's ozone layer, they have been replaced by hydrofluorocarbons (HFCs), which do not react with ozone.

In one brand of inhalers, the original CFC propellant has been replaced by HFC 227ea (C₃HF₇,

hepta?uoropropane). The volume of the inhaler propellant reservoir is $\text{l}.00\times {10}^2\text{mL}$, and the propellant is charged into the reservoir to a gauge pressure of 4.443 atm at 23°C. An online search for properties of HFC 227ea yields the information that the critical temperature and pressure of the substance are 374.83K and 28.74 atm, and the acentric factor is $\omega =0.180$.

(a) Assuming ideal-gas behavior, estimate the mass(g) of propellant in the fully charged inhaler.

(b) Someone in the manufacturer's Quality Control Division has raised a concern that assuming ideal-gas behavior might be inaccurate at the charging pressure. Use the SRK equation of state to recalculate the moles of propellant at the speci?ed conditions. What percentage error resulted from using the ideal-gas assumption?