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@X@user.full_name@X@ @GMU: Uranium -238 decays in a series of steps to Pb-206. When the material is very old, one can treat the kinetics as a decay directly to Pb and the ratio of these two can be treated as a measure of the age of a rock. A rock containing 238U and 206Pb was examined to determine its approximate age. The molar ratio of 206Pb to 238U was 0.1852. Assuming no lead was originally present and that all lead formed remained in the rock, what is the age of the rock (in years)? The half-life of 238U is 4.50e9 yr.

@X@user.full_name@X@ @GMU: What would the pH be after addition of 19.15 mL of 0.1000 M NaOH to 25.00 mL of 0.1000 HA (a weak acid, Ka = 3.71e-4)?

@X@user.full_name@X@ @GMU: Determine the concentration of A after 94.514 seconds if the initial concentration of A was 0.2021 M and the first order rate constant is 4.176e-3 s-1.

John Schreifels @GMU: When 8.078 g of citric acid was dissolved in 104.5 g of benzene, the freezing point was 276.6 K. What is the formula mass of this compound? The freezing point depression constant for benzene is 5.12 (°C*kg)/mol. The melting point and boiling points are 5.50°C and 80.10°C, respectively.

@X@user.full_name@X@ @GMU: What is the temperature at which the reaction below becomes spontaneous, if the standard entropies of reactants and products at 25oC are So (C(gr)) = 5.74 J/mol*K, So(H2(g)) = 130.68 J/mol*K, and So (C3H6(g)) = 266.6000J/mol*K and the heat of formation of C3H6(g)) is 20.4200 kJ/mol?3C(gr) + 3H2(g) → C3H6(g)

@X@user.full_name@X@ @GMU: The reaction below occurs in the electrochemical cell. Which answer is correct?2Fe3+(aq) + 3Cd(s) → 2Fe(s) + 3Cd2+(aq)

@X@user.full_name@X@ @GMU: What would the pOH be after addition of 6.72 mL of 0.1000 M NaOH to 25.00 mL of 0.1000 HA (a weak acid, Ka = 5.51e-4)?

@X@user.full_name@X@ @GMU: What is the solubility in g/L of O2 at a partial pressure of 311.9 torr, if the solubility at a partial pressure of 518.4 torr is 0.03012 g/L?

John Schreifels @GMU: What is the change in mass of 95Mo when it is formed from protons, electrons and neutrons if the atom has a mass of 94.9058411 amu? The masses of the proton, electron and neutron are 1.00728 amu, 0.000549 amu and 1.00867 amu, respectively.

@X@user.full_name@X@ @GMU: Calculate the [H3O+] of 119.11 mL of a buffer initially consisting of 0.1104 M C2H5NH2 and 0.1023 M C2H5NH3NO3 after addition of 0.0021 mol of HCl. Assume that no volume change occurs after addition of the acid. The Kb of C2H5NH2 is 5.60e-4