Both of these problems use the ideal gas law. pV=nRT. p = pressure in atm, V = volume in L, n = number of mols of gas, R = gas constant (0.08206) and T = temperature in kelvins.
#13) 1(atm)*10L = n(0.08206)(298) n = 0.4089 mols.
the molar mass of methane is roughly 16 g/mol and therefore 16*0.4089 = 6.5g.