@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 0.1807. 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: RbCl has a face-centered cubic unit cell in which the Cl- anions occupy corners and face centers, while the cations fit into the hole between adjacent anions. What is the radius of Cl- if the ionic radius of Rb+ is 148.7 pm and the density of RbCl is 2.800 g/cm3?

@X@user.full_name@X@ @GMU: What is the rate constant at 500.54°C for the reaction below if the rate constant is 1.80e-4 s-1 at 477.00°C and the activation energy is 270.00 kJ/mol?Cyclopropane → Propene

@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 (C3H4(g)) = 248.2000J/mol*K and the heat of formation of C3H4(g)) is 185.5200 kJ/mol?3C(gr) + 2H2(g) → C3H4(g)

@X@user.full_name@X@ @GMU: Calculate the [H3O+] of 112.82 mL of a buffer initially consisting of 0.1756 M NH2OH and 0.1784 M NH3OHBr after addition of 0.0026 mol of HCl. Assume that no volume change occurs after addition of the acid. The Kb of NH2OH is 9.10e-9

@X@user.full_name@X@ @GMU: What is the solubility in mol/L of N2 at a partial pressure of 388.2 torr, if the solubility at a partial pressure of 522.1 torr is 0.0004740 mol/L?

@X@user.full_name@X@ @GMU: When 0.2699 g of lysine was dissolved in 101.5 mL of water, the osmotic pressure at 10.00°C was 0.4231 atm. What is the formula mass of this compound?

In the lung’s alveoli, there are four gases mixed together: O2, carbon dioxide, nitrogen, and water vapor. At a measured barometric pressure of 755 mmHg, alveolar O2 exerts a partial pressure of 100 mmHg, CO2 44 mmHg, and water vapor 47 mmHg. What is the alveolar partial pressure of nitrogen?

What is the external marker of the point where the trachea divides into the left and right main stem bronchi?

In patients with closed-head injuries, what may happen in the presence of hypercapnea?