Chemistry Year in Review

What is the correct noble gas notation for Vanadium (V)?

• [Ne] 3s²3p⁶4s²3d³

• [Kr] 4s²3d³

• [Ar] 4s²3d³

• [Ar] 3d⁵

Answer explanation

Vanadium (V) has an atomic number of 23. The noble gas notation starts with the nearest noble gas, Argon [Ar], followed by the electron configuration in the 4s and 3d orbitals, which is 4s²3d³.

Tags

NGSS.HS-PS1-1

Some students are discussing the description of an atom. One student describes it this way. "Negatively charged electrons in a cloud surround a nucleus composed of positively charged protons, with most of the atom's mass in the electron cloud." Which statement best evaluates the student's description of atomic structure?

• The description is completely correct as electrons are negative, protons are positive, and the mass is concentrated in the electron cloud.

• The description is partially correct but fails to mention neutrons in the nucleus and incorrectly states where most of the mass is located.

• The description is completely incorrect as electrons are positive, protons are negative, and the mass is evenly distributed.

• The description is only correct about electron charge but wrong about proton location and atomic mass distribution.

Answer explanation

The student's description is partially correct as it accurately states the charges of electrons and protons. However, it omits neutrons in the nucleus and incorrectly claims that most of the atom's mass is in the electron cloud.

The mass of an atom of Chlorine-35 is 35.0 u. What is the mass of the same atom when it gains an electron to become a chloride ion?

• 34

• 35

• 36

• 37

Answer explanation

When Chlorine-35 gains an electron to become a chloride ion, its mass remains essentially the same at 35.0 u. The added electron has negligible mass, so the correct answer is 35.

Tags

NGSS.HS-PS1-7

An electron dot diagram for Lithium should include:

• one valence e-dot

• two valence e-dots

• 3 valence e-dots

• 7 valence e-dots

Answer explanation

Lithium has one valence electron, which is represented by one e-dot in its electron dot diagram. Therefore, the correct answer is one valence e-dot.

Tags

NGSS.HS-PS1-1

How many molecules are present in 2.5 moles of water (H₂O)?

• 6.02 × 10²³ molecules

• 1.51 × 10²⁴ molecules

• 2.41 × 10²³ molecules

• 3.01 × 10²³ molecules

Answer explanation

To find the number of molecules in 2.5 moles of water, multiply by Avogadro's number (6.02 × 10²³). Thus, 2.5 moles × 6.02 × 10²³ = 1.51 × 10²⁴ molecules, which is the correct answer.

A sample contains 3.01 × 10^24 atoms of copper. How many moles of copper are present?

• 5.0 moles

• 2.0 moles

• 1.0 mole

• 0.5 mole

Answer explanation

To find moles, use Avogadro's number (6.02 × 10^23 atoms/mole). Divide 3.01 × 10^24 atoms by 6.02 × 10^23 atoms/mole, which equals 5.0 moles. Thus, the correct answer is 5.0 moles.

Which term represents the simplest whole-number ratio of atoms of the elements in a compound?

• atomic mass

• formula mass

• empirical formula

• structural formula

Answer explanation

The empirical formula represents the simplest whole-number ratio of atoms of the elements in a compound, distinguishing it from the molecular formula, which may show the actual number of atoms.

What makes water a polar molecule?

• Equal sharing of electrons between hydrogen and oxygen atoms

• Uneven distribution of electrons, creating partial positive and negative charges

• Complete transfer of electrons from hydrogen to oxygen

• The linear arrangement of atoms in the water molecule

Answer explanation

Water is a polar molecule due to the uneven distribution of electrons between the hydrogen and oxygen atoms. This creates partial positive charges on the hydrogen atoms and a partial negative charge on the oxygen atom.

A student needs to prepare 500 mL of a 0.350 M calcium chloride (CaCl₂) solution. How many moles of CaCl₂ should the student use?

• 0.0700 moles

• 0.175 moles

• 0.350 moles

• 0.700 moles

Answer explanation

To find the moles of CaCl₂ needed, use the formula: moles = molarity × volume (L). Here, 0.350 M × 0.500 L = 0.175 moles. Thus, the student should use 0.175 moles of CaCl₂.

What is conserved during the dilution of a solution?

• Molarity of the solution

• Volume of the solution

• Total moles of solute

• Mass of the solvent

Answer explanation

During dilution, the total moles of solute remain constant, as only the volume and molarity change. Therefore, the correct answer is that the total moles of solute are conserved.

Tags

NGSS.MS-PS1-5