InterviewSolution
Saved Bookmarks
InterviewSolution
| 1. |
Molecular mass from osmotic pressure: Pepsin is an enzyme present in the human digestive tract. A solution of a `0.500 gram` sample of purified pepsin in `30.0 mL` of aqueous solution exhibits an osmotic pressure of 892 torr at `27^(@)C`. Estimate the molecular mass of pepsin. Strategy: An enzyme is a protein that acts as a biological catalyst. Pepsin catalyzes the metabolic cleavage of amino acid chain (called peptide chains) in other proteins. To determine molecular mass, we must first find the number of moles `(n)` of pepsin represented by the `0.500 g` sample. We can do this by first rearranging the equation `pi=CRT` for osmotic pressure to find the molarity of the pepsin solution and then multiplying it by the volume of the solution to obtain the number of moles of pepsin. |
|
Answer» Step 1. Calculate molarity of pepsin Since `pi =CRT`, then `C=pi/(RT)` `C=((8.92 "torr" xx(1 "atm")/(760 "torr")))/((0.082 (L "atm")/(K mol))(300 K))` `=0.000477 mol L^(-1)` `=4.77xx10^(-4) mol L^(-1)` Note that we convert 8.92 torr to atmospheres to be consistent with units of R. Step 2. Calculate moles of pepsin `Molarity (C )=n_("pepsin")/V_(mL)xx(1000 mL)/L` Since the volume of the solution is `30.0 mL`, the number of moles of pepsin is `n_("pepsin")=(4.77xx10^(-4) mol L^(-1))(30.0 mL)((1L)/(1000 mL))` `=143.1xx10^(-7) mol` `1.43xx10^(-5) mol` Step 3. Calculation the molar mass Knowing both the mass and the number of moles of pepsin, we can calculate the molar mass and hence molecular mass `"moles"_("pepsin")=("mass"_("pepsin"))/("molar mass"_("pepsin"))` or `"molar mass"_("pepsin")=("mass"_("pepsin"))/("moles"_("pepsin"))` `=(0.500 g)/(1.43xx10^(-5) mol)` `=0.349xx10^(5) g mol^(-1)` `=3.50xx10^(4) g mol^(-1)` Thus, the molecular mass of pepsin is approximately 35,000 amu. This is typical for medium-sized proteins. Alternatively, we can calculate the molar mass of pepsin through the equation `(2.74)`. |
|