What is the total number of electrons that can occupy the third principal quantum shell (n = 3)?

The maximum number of electrons ($N$) that can occupy a principal quantum shell with quantum number $n$ is given by the formula $N = 2n^2$.

The final answer is $\boxed{\text{D. 18}}$.

2. Which mass of sodium hydroxide, NaOH, is required to prepare 250cm³ of a 0.200mol/dm³ solution? (M. M of NaOH = 40.0)

Moles = Concentration × Volume

Mass = Moles × Molar Mass

The final answer is $\boxed{\text{B. 2.00g}}$.

3. Which of the following molecules is polar?

• A. BeCl_2: Beryllium chloride has a linear geometry. Although the Be-Cl bonds are polar, the two dipoles are equal in magnitude and opposite in direction, causing them to cancel out. The molecule is nonpolar.
• B. BCl_3: Boron trichloride has a trigonal planar geometry. The B-Cl bonds are polar, but due to the symmetrical arrangement, the three dipoles cancel each other out. The molecule is nonpolar.
• C. NCl_3: Nitrogen trichloride has a trigonal pyramidal geometry. The N-Cl bonds are polar, and the nitrogen atom has a lone pair of electrons, which makes the molecule asymmetrical. The bond dipoles do not cancel out, resulting in a net dipole moment. The molecule is polar.
• D. CCl_4: Carbon tetrachloride has a tetrahedral geometry. The C-Cl bonds are polar, but the symmetrical arrangement of the four bonds causes the dipoles to cancel out. The molecule is nonpolar.

The final answer is \boxed{\text{C. NCl_3}}.

4. Why does a solution have a lower rate of evaporation than a pure solvent?

The presence of solute particles in a solvent leads to a lower rate of evaporation primarily because the solute particles attract the solvent molecules more strongly. This increased attraction makes it more difficult for solvent molecules to escape from the liquid phase into the gas phase, thus reducing the vapor pressure and the rate of evaporation. Additionally, solute particles occupy some of the surface area, reducing the number of solvent molecules available to evaporate.

The final answer is $\boxed{\text{C. The solute particles attract the solvent molecules more strongly}}$.

5. What is the relative atomic mass of an element with two isotopes: X-10 (20% abundance) and X-11 (80% abundance)?

Relative atomic mass is calculated as the sum of (isotope mass × fractional abundance) for all isotopes.

For X-10 (20% abundance): $10 \times 0.20 = 2.0$
For X-11 (80% abundance): $11 \times 0.80 = 8.8$

The final answer is $\boxed{\text{B. 10.8}}$.

6. When an atom is oxidized, its oxidation number

Oxidation is defined as the loss of electrons. When an atom loses negatively charged electrons, its overall charge becomes more positive. Therefore, its oxidation number increases.

The final answer is $\boxed{\text{B. Increases as electrons are lost}}$.

7. Which of the following compound's shape is not obtained using VSEPR theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict the geometry of covalent molecules based on the repulsion between electron pairs around a central atom.

• A. Boron trifluoride (BF_3): This is a covalent molecule with a central boron atom. VSEPR theory predicts its trigonal planar shape.
• B. Sodium Chloride (NaCl): This is an ionic compound. It exists as a crystal lattice of Na⁺ and Cl⁻ ions, not as discrete molecules with a specific shape predicted by VSEPR theory. VSEPR theory does not apply to ionic compounds.
• C. Methane (CH_4): This is a covalent molecule with a central carbon atom. VSEPR theory predicts its tetrahedral shape.
• D. Carbon dioxide (CO_2): This is a covalent molecule with a central carbon atom. VSEPR theory predicts its linear shape.

The final answer is $\boxed{\text{B. Sodium Chloride}}$.

8. Which atomic structure is described as a diffuse cloud of positive charge with negative electrons embedded around it

The atomic model described as a diffuse cloud of positive charge with negative electrons embedded within it is the Plum Pudding Model. This model was proposed by J.J. Thomson in 1904.

The final answer is $\boxed{\text{B. Plum pudding model}}$.

9. In acid-base titration, solid sodium hydroxide is not a primary standard because it:

A primary standard must be a highly pure, stable, non-hygroscopic substance that can be weighed accurately. Sodium hydroxide (NaOH) is hygroscopic, meaning it readily absorbs moisture from the atmosphere. It also reacts with carbon dioxide in the air. These properties cause its mass to change upon exposure to air, making it impossible to weigh accurately and prepare a solution of precisely known concentration.

The final answer is $\boxed{\text{D. absorbs moisture from the atmosphere}}$.

10. On a phase diagram for a substance like Carbon Dioxide, if you increase pressure while at a constant high temperature, which phase transition are you likely to trigger?

A phase diagram illustrates the conditions of temperature and pressure at which a substance exists in different phases (solid, liquid, gas).

For most substances, increasing pressure at a constant temperature tends to favor the denser phase. If a substance is in the gaseous phase at a high temperature, increasing the pressure will force the gas molecules closer together. This can lead to a transition from the gaseous state to the liquid state. This transition is known as condensation. For CO₂, at temperatures above its triple point but below its critical temperature, increasing pressure from a gaseous state will cause it to condense into a liquid.

The final answer is $\boxed{\text{C. Condensation}}$.