Explanation:
Relative atomic mass of Br=(0.5069)(78.92)+(0.4931)(80.92)=79. The rest of the choices are incorrect relative atomic masses of bromine.

Alternative Method:

79Br atomic mass can be rounded up to 79

81Br atomic mass can be rounded up to 81

Since the relative abundance of both isotopes is about 50% each, the relative atomic mass of Bromine should be close to 80. Choice D corresponds to that estimation.

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:
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 electron configuration for S is 1s2 2s2 2p6 3s2 3p4, whereas for $\mathrm{P}$, it is 1s2 2s2 2p6 3s2 3p3.
Due to the increased stability of P’s half-filled p-shell, more energy is required to remove an electron from a $\mathrm{P}$ atom than an $\mathrm{S}$ atom. The electron-electron repulsion in Sulfur makes it easier to remove the first electron.
First ionization energy increases across a period (due to a decrease in atomic radii and an increase in nuclear charge). In period 3, Na is the first element, and Ar is the last to the right.

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:
1 mole is the number of H2 molecules in 2.02 g of H2. Choices A, B, and C all conform to the definition of a mole

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

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

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

Explanation:
The s orbital has a spherical shape. In terms of energy, s has the lowest, followed by p and d orbital. Thus, Choices A, B, and D are all incorrect

Explanation:
Seawater is a mixture of water and salts. Sucrose, distilled water, and sodium chloride are all examples of compounds.

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:
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:
Ions do not have the same numbers of protons and electrons. Only species B and C do not have the same number of protons and electrons, so the correct answer is B.

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:
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.