Why does calcium have a larger atomic radius than magnesium

Down the group , atomic radius of atoms increase due to increase in number of shells there by increasing shielding effect and decreasing effective nuclear charge . ( effective nuclear charge pulls the valence shell closer to the nucleus)

in the given question, the partial pressure is calculated by using the ideal gas equation, that is, pv = nrt

here, p is pressure, v is volume = 25 l, n is number of moles = 0.0104 moles, r is gas constant = -0.0821 l.atm / mol.k

t is temperature = 273 k

p = nrt / v

p = (0.0104 mol × 0.0821 l.atm / mol.k × 273 k) / 25 l

= 0.0093 atm

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.