Explanation​:

The atomic number is the number of protons. 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​:
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​:
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​:
An insoluble solid can be separated from a liquid by means of filtration. Distillation is used to separate homogeneous liquid mixtures (solid soluble in a liquid or two miscible liquids) with different boiling points. Sublimation is a technique for separating mixtures with volatile components that can undergo sublimation. Crystallisation is a separation technique that uses solubility properties to separate a solid from a solution.

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

Explanation:​
The $\mathrm{Mn}$ atom and Mn^2+ ion have the following orbital diagrams:

The valence electrons and the two lost in the 2+ ion are the $4s$ electrons so the unpaired electrons remain 5.

Explanation​:

Isotopes with fewer neutrons will have a lower density

Explanation:
Add $273$ to the temperature in Celsius to convert it to Kelvin. −185∘C+273=88 K. The rest of the choices are incorrect.

Explanation​:
Liquids have an indefinite shape (take the shape of the container) but have a definite volume. Choice A describes solids. Choices B and D describe the characteristics of gases.

Explanation:
Cu has the electron configuration [Ar] 4s13d10 (a full d-subshell is more stable). The $4s$ electron is lost in the first ionization followed by a $3d$ electron.

Explanation:
The graph of P vs T shows a linear graph (straight line passing through the origin) that rises upward. This graph shows that P is directly proportional to the absolute temperature. As the absolute temperature increases, the pressure of the gas also increases.

Explanation​:
The empirical formula shows the simplest whole-number ratio of elements in a compound. The empirical formula can sometimes be the same as the molecular formula. Choices A, B, and D are all false statements.

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:
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​:
To answer this question, the student must understand the properties of the mixture's components.
The wood chips would not dissolve in the water, so they are filtered out first. This will leave the sodium chloride dissolved in the water. The water can be evaporated to remove the water (to isolate the salt).
Paper chromatography would be ineffective in this scenario as it is used for separating substances such as dyes, pigments, and proteins.
This question is categorized as medium complexity because the student must understand the properties of matter and separation techniques.

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​:

Electrons have negligible mass.

Explanation:
The molar mass of sodium phosphate is 163.94 g mol^−1. Divide the mass ($20.5g$) of sodium phosphate by the molar mass to calculate the number of moles of the compound. The rest of the choices are incorrect.