Explanation​:
Based on the balanced chemical equation, the number of moles of NO2​ is twice that of N2O4. Avogadro's law states that the volume of a gas is directly proportional to the number of moles of the gas present (25/2= 12.5).

Explanation​:
The student must first understand that each component of the mixture will result in one spot on the paper chromatogram. If the recrystallization was successful, then the purified dye should only have one dot.
Therefore, answer choice B is incorrect since the sample contains more than one component after recrystallization.
Answer choice C indicates that the dye is no longer in the sample, and answer choice D indicates the contaminant was not successfully separated from the dye. So the paper chromatogram is unable to identify any impurities that are still present.
In answer choice A, before recrystallization, shows two components (the dye and contaminant), and after, there is only one component showing successful recrystallization.
This question is categorized as medium since the student must understand the premise and purpose of paper chromatography and apply it to experimental results

Explanation:
Explanation: Selenium's electron configuration is 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^4. The three $2p$ and three $3p$ orbitals are fully occupied, along with one 4p orbital.

Explanation:​
Electrons fill sublevels according to the Aufbau Principle shown below: 

Following this order, 2p<4s<3d<4p

Explanation​:

Isotopes are atoms of the same element with different numbers of neutrons that differ only in terms of their physical properties.

Explanation​:
At high pressures and very low temperatures, the inter-particle forces strengthen, spaces between particles decrease causing the deviation from ideal gas behaviour PV/nRT​ > 1.

Explanation​:
The lines converge as energy levels increase (increased distance from the nucleus).
Higher energies result in higher frequencies.

The mass number is the sum of the number of protons and neutrons. For a neutral atom, the number of electrons is equal to the number of protons

Explanation​:
Successive ionization energies are larger, as electrons are removed, there is less electron-electron repulsion, so the remaining electrons are pulled closer to the nucleus, and more energy is required to remove them.

Explanation:

Isotopes with fewer neutrons will have a lower density

Explanation:
The shortest wavelength corresponds to a higher frequency and therefore a higher energy transition. The 5p to $1s$ represents the largest energy drop and therefore would produce the shortest wavelength.

Explanation:
The student must first recall that the component with the greatest affinity for the mobile phase will travel the furthest in paper chromatography. In this case, spot #2 is the furthest from the starting line, so it has the greatest affinity for the mobile phase.
In paper chromatography, the mobile phase is always the solvent (water, in this case), while the stationary phase is the paper.
This question is categorized as easy because the student must only recall basic definitions and concepts for paper chromatography.

Explanation​:
Electron configuration of the $\mathrm{Cr}$ atom is [Ar] 4s13d^5.
The loss of one s and two d electrons results in [Ar] 3d^3 configuration.

Explanation​:
Choice A is incorrect because the orbital diagram does not satisfy Hund's rule wherein all orbitals must be singly occupied before pairing.
Choice B is incorrect because Pauli's Exclusion principle is not obeyed by having two electrons with the same spin in one orbital.
Choice C is correct because, according to Hund's rule, the orbitals must be half-filled first before pairing, and the paired orbital must have electrons in an opposite spin.
Choice D is incorrect because the 2s orbital must be filled first before filling the 2p orbital, as illustrated by Aufbau's Principle.

Explanation:

Electrons have negligible mass.

Explanation:
For p, $l$ is equal to 1. The number of electrons in a given subshell can be calculated using the formula: $6$ electrons.

The p subshell holds 3 orbitals. Each orbital can fit 2 electrons. Therefore, p subshell holds 6 electrons.

Explanation​:
There is a large increase between the 3rd and 4th ionization energies which shows that the element is in group 13.
Large increases in ionization energy indicate electrons being removed from a shell closer to the nucleus.

Explanation​:
Statement I is true because emission spectra are produced when photons are released from excited electrons returning to lower energy levels.
Statement II is true because this is Pauli’s Exclusion principle.
Statement III is also true because it is included in Pauli’s Exclusion principle.

Explanation:
Isotopes have the same number of protons but different numbers of neutrons.
Species A and B have the same number of protons but also the same number of neutrons so cannot be isotopes.
Species C is the only species in the table with 9 protons so cannot have an isotope with another species in the table.
This leaves species D and E which both have the same number of protons but a different number of neutrons. Therefore, the correct answer is D.

Explanation:​
Condensation (gas to liquid) requires making bonds and therefore, releases energy. Sublimation (solid to gas) requires breaking bonds and therefore absorbs energy.

Explanation​:
One of the laws of gases states that, for a fixed amount of gas held at a constant temperature, the volume of a gas is inversely proportional to its pressure.

Explanation​:
The atomic number of sodium is 11. The atom loses one electron to become a stable ion with a charge of 1+ (10 electrons).
Choice A is incorrect because it shows the electron configuration of a sodium atom, not sodium ion. Choice B is incorrect because it shows an increase instead of a decrease by 1 electron.
Choice D is incorrect because of 2s^1.
Choice C is the correct answer. Ne has an atomic number 10. In a neutral atom, the number of electrons is equal to the number of protons. Therefore, Ne has 10 electrons in its atom. Sodium ion has 10 electrons too.

Explanation​:
Choice B is incorrect because electrons are not located in the nucleus. Choice C is incorrect because an orbital diagram is just a representation of electrons in an orbital. Choice D is incorrect because electrons can be found in orbit (a specific energy level around the positively charged nucleus) but the probability is relatively low. Choice A is the correct answer: orbitals are areas/ regions in space where the probability of finding an electron is very high.