🧪AP Chemistry
2. Answer the following questions about calcium and its compounds.
A mass spectrum for a naturally occurring sample of calcium is shown in Figure 1.
Figure 1. Mass spectrum of naturally occurring calcium (relative abundance by mass number).
Describe the atomic structure of the calcium-40 isotope in terms of the number of protons and neutrons.
Using the data in Figure 1, explain why the average atomic mass of calcium is 40.08 amu, which is very close to the mass of the calcium-40 isotope.
A student performs an experiment to determine the empirical formula of a calcium compound formed by reacting calcium metal with nitrogen gas, . The student heats a sample of calcium in a crucible containing nitrogen gas. The data from the experiment are presented in Table 1.
Table 1. Data from gravimetric analysis
Measurement | Mass (g) |
|---|---|
Mass of empty crucible | 20.00 |
Mass of crucible + calcium | 21.20 |
Mass of crucible + product (after heating) | 21.48 |
Calculate the number of moles of calcium used in the experiment based on the data in Table 1.
Calculate the number of moles of nitrogen that reacted with the calcium.
Determine the empirical formula of the calcium nitride compound formed. Justify your answer using the mole calculations from parts B(i) and B(ii).
The student analyzes the electron structure of calcium using Photoelectron Spectroscopy (PES). The spectrum is shown in Figure 2.
Figure 2. Photoelectron spectrum (PES) of calcium showing electron binding energies and relative numbers of electrons per sublevel.
Based on the electron configuration of a neutral calcium atom, identify the peak in Figure 2 that corresponds to the electrons in the 3p sublevel. Justify your answer in terms of relative energy and number of electrons.
The peak for the 4s sublevel in the spectrum of calcium (Z=20) has a higher binding energy than the peak for the 4s sublevel in the spectrum of potassium (Z=19). Explain this difference in terms of atomic structure.
Calcium reacts with oxygen to form calcium oxide, CaO. Potassium reacts with oxygen to form potassium oxide, .
Which compound, CaO or , has the higher melting point? Justify your answer using Coulomb's law.
The calcium sample used in the experiment in part B was contaminated with a significant amount of magnesium. Does this error cause the calculated number of moles of nitrogen (part B(ii)) to be greater than, less than, or equal to the actual moles of nitrogen that would react with a pure sample of calcium of the same mass? Justify your answer.
Explain why the second ionization energy of calcium is greater than the first ionization energy, but significantly less than the third ionization energy.
