# Finding Pressure of M in a Charge Exchange

Ar (RE = 15.5 eV) is the reagent gas in a charge exchange experiment. If Ar+ reacts with anisole with a rate constant of 2.26x10-10cm3 molecules-1/s-1 and 65% of the Ar ions react in 25 milliseconds, what is the PRESSURE of anisole if the temperature of the ion source is 25 C? The IE of anisole is 7.78eV.

Not exactly what to do here. I know that -Hrxn=RE-IE, meaning that -Hrxn=7.72eV. Converting this would be -Hrxn=749.21kJ/mol. How would you go about using the rest of the information given along with this -Hrxn to find the pressure?

• 65% is about the amount consumed in one turnover, i.e. $1 - e^{-1} \approx 0.65$. Therefore, maybe you could just multiply the rate constant times the 25 milliseconds to get a number for mL per molecule. Multiplying by the gas constant $R$ converts that to a molar volume $V$ with units of moles / volume. Then the ideal gas law is $P = R T / V$. The only assumption I think is that anisole is in vast excess relative to $\ce{Ar+}$. – Curt F. Mar 18 '19 at 23:41