Question

1) Leucomethylene blue is known to be re-oxidized by dissolved O₂ (from the air) back to methylene blue. Under acidic conditions the oxidation of leucomethylene blues is slow while the reduction of methylene blue is fast. This ensures that the complication of leucomethylene blue oxidation is largely avoided in our experiments (which were performed at low pH). Nevertheless, briefly discuss how the observed kinetics of methylene blue reduction would be affected if the reverse reaction (HMB oxidation) was significant (e.g., at higher pH). Suggest how the rate law (equation 3 which is -d[MB⁺]/dt=k_obs [MB⁺ ]^a would need to be modified to take this into account, assuming that the oxidation is first order in HMB ad that dissolved O₂ is in large excess over HMB.

2) Prepare ~4 mL of ascorbic acid stock solution by weighing out the appropriate amount of solid into a 15 mL plastic Falcon tube and dissolving it in the appropriate amount of deionized water. The molar mass of ascorbic acid is 176 g/mol, so a 250 mM stock solution should contain 44 mg ascorbic acid per mL of water. The concentration needs to be fairly accurate so adjust the amount of water you add acccording to the amount of ascorbic acid you weigh out in order to achieve a 250 mM solution. Please help me determine how many grams of ascorbic acid I need to prepare this solution.

3) How can you find the pH from given  pKa value and how can you determined the predominant species. For example they give you monoprotonated leucomethylene blue with given pKa value 5.9. How would you do that?

Any questions you can help me with, will be find. Please help me, I am begging you, I really need those answer.

Answer 1

1) the rate law will be

2) if you only need to prepare 4 mL of 250 mM solution. you need 176 mg.

     mass = (M)(V)(MM) = (0.250M)(0.004L)(176g/mol) = 0.176 g or 176 mg

3) Use the Henderson Hasselbalch equation, pH = pKa + log ([conjugate base]/[acid]). Predominant species can be determined if pH and pKa are given. If pH is lower than pKa, the acidic species is more dominant, while if pH is higher than pKa the basic species will be more dominant.