How many atoms are present in 12.0 g of magnesium? (Mg = 24, NA = 6.02 x 10²³ mol⁻¹) A gas was collected over water at 25°C and 740 mm Hg. Calculate the volume of the gas at STP if the saturated vapou

Number of moles ($n$) = $\frac{\text{Mass}}{\text{Molar mass}}$
Given mass = 12.0 g
Molar mass of Mg = 24 g/mol

Number of atoms = Number of moles $\times$ Avogadro's number (NA)
Given NA = $6.02 \times 10^{23} \text{ mol}^{-1}$

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

22. A gas was collected over water at 25°C and 740 mm Hg. Calculate the volume of the gas at STP if the saturated vapour pressure of water at 25°C is 24 mm Hg. The gas volume collected is 100 cm³.

When a gas is collected over water, the total pressure is the sum of the partial pressure of the dry gas and the vapor pressure of water.

Given $P_{\text{total}} = 740 \text{ mm Hg}$ and $P_{\text{water vapor}} = 24 \text{ mm Hg}$.

The combined gas law is $\frac{P_1V_1}{T_1} = \frac{P_2V_2}{T_2}$.
Initial conditions (1):
$P_1 = 716 \text{ mm Hg}$
$V_1 = 100 \text{ cm}^3$
$T_1 = 25^\circ\text{C} = 25 + 273 = 298 \text{ K}$
Standard Temperature and Pressure (STP) conditions (2):
$P_2 = 760 \text{ mm Hg}$ (standard pressure)
$T_2 = 0^\circ\text{C} = 0 + 273 = 273 \text{ K}$
$V_2 = ?$

Rounding to one decimal place, $V_2 = 86.4 \text{ cm}^3$.

The final answer is $\boxed{\text{A}}$

23. In the reaction $2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{g})$, if 500 cm³ of hydrogen reacts with 200 cm³ of oxygen in a sealed tube, what volume of hydrogen would remain unreacted?

From the balanced equation, 2 volumes of H₂ react with 1 volume of O₂.
Given:
Volume of H₂ = 500 cm³
Volume of O₂ = 200 cm³

If 1 volume of O₂ reacts with 2 volumes of H₂, then 200 cm³ of O₂ will react with $2 \times 200 \text{ cm}^3 = 400 \text{ cm}^3$ of H₂.

Initial volume of H₂ = 500 cm³
Volume of H₂ reacted = 400 cm³
Volume of H₂ unreacted = Initial volume - Reacted volume

The final answer is $\boxed{\text{A}}$

24. Which of the following phenomena provide evidence for the particulate nature of matter?
The particulate nature of matter refers to the idea that matter is made up of tiny, discrete particles (atoms, molecules, ions) that are in constant motion.

• I. Evaporation: Particles gain enough energy to escape from the liquid phase into the gas phase, demonstrating particle movement.
• II. Sublimation: Particles transition directly from solid to gas, also demonstrating particle movement and discrete nature.
• III. Diffusion: Particles spread out from an area of high concentration to low concentration due to their random motion and collisions.
• IV. Brownian motion: The random movement of visible particles suspended in a fluid, caused by collisions with the invisible, rapidly moving fluid particles. This is direct evidence of the constant random motion of particles.

All four phenomena provide evidence for the particulate nature of matter.

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

25. Elements P and Q have atomic numbers 6 and 8, respectively. What type of bond will they most likely form, and what is the formula of the resulting compound?

Element P: Atomic number 6. Electron configuration: $1s^2 2s^2 2p^2$. It has 4 valence electrons. It needs 4 electrons to complete its octet.
Element Q: Atomic number 8. Electron configuration: $1s^2 2s^2 2p^4$. It has 6 valence electrons. It needs 2 electrons to complete its octet.

Both P (carbon) and Q (oxygen) are non-metals. Non-metals typically form covalent bonds by sharing electrons to achieve a stable octet.

P needs 4 electrons, and Q needs 2 electrons. To satisfy their octets, one atom of P will share electrons with two atoms of Q.
P (valency 4) and Q (valency 2).
The least common multiple of 4 and 2 is 4.
So, 1 atom of P combines with 2 atoms of Q.
The formula will be PQ₂. (For example, carbon dioxide, CO₂).

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

26. An ion M²⁺ has 10 electrons and 12 neutrons. What is the mass number of element M?

An ion M²⁺ means the element M has lost 2 electrons.
If M²⁺ has 10 electrons, then the neutral atom M must have had $10 + 2 = 12$ electrons.
In a neutral atom, the number of electrons is equal to the number of protons.
So, element M has 12 protons. The atomic number of M is 12.

Mass number = Number of protons + Number of neutrons
Given number of neutrons = 12.
Mass number = $12 \text{ (protons)} + 12 \text{ (neutrons)} = 24$.

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

27. Which of the following electronic configurations represents an atom in an excited state?
An excited state occurs when an electron moves from a lower energy level to a higher energy level, leaving a vacancy in the lower level. The ground state configuration is the lowest energy arrangement of electrons.

• A. $1s^2 2s^2 2p^6$: This is the ground state configuration for Neon (atomic number 10). All orbitals are filled according to Aufbau principle.
• B. $1s^2 2s^2 2p^5 3s^1$: This configuration shows an electron from the $2p$ orbital promoted to the $3s$ orbital. The $2p$ orbital is not fully filled before electrons occupy the $3s$ orbital. This represents an excited state.
• C. $1s^2 2s^2 2p^6 3s^2$: This is the ground state configuration for Magnesium (atomic number 12).
• D. $1s^2 2s^1$: This is the ground state configuration for Lithium (atomic number 3).

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