Which of the following elements must readily reacts with water? Which of the following pH values is most likely to be that of a weak acid? Of the first 18 elements in the periodic table, how many are gases at 25 °C and 100 kPa?

Here are the answers to the multiple-choice questions:

1. Which of the following elements most readily reacts with water?

• Alkali metals (Group 1) are known for their vigorous reaction with water, and reactivity increases down the group.
• Lithium and Potassium are both alkali metals. Potassium is below Lithium in Group 1, making it more reactive. Fluorine and Chlorine are halogens and react differently.
• Therefore, Potassium reacts most readily with water among the given options.

The correct answer is D. Potassium.

2. Which of the following pH values is most likely to be that of a weak acid?

• A strong acid has a very low pH (e.g., 1 or 2).
• A weak acid has an acidic pH (less than 7) but is closer to 7 than a strong acid.
• pH 8 is basic. pH 1 and 2 are characteristic of strong acids. pH 6 is acidic but close to neutral, which is typical for a weak acid.

The correct answer is C. 6.

3. Of the first 18 elements in the periodic table, how many are gases at 25 °C and 100 kPa?

• The first 18 elements are: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar.
• At 25 °C and 100 kPa (standard conditions), the elements that are gases are:
  • Hydrogen ($\text{H}_2$)
  • Helium ($\text{He}$)
  • Nitrogen ($\text{N}_2$)
  • Oxygen ($\text{O}_2$)
  • Fluorine ($\text{F}_2$)
  • Neon ($\text{Ne}$)
  • Chlorine ($\text{Cl}_2$)
  • Argon ($\text{Ar}$)
• Counting these, there are 8 gases.

The correct answer is C. Eight.

4. The following reaction produces a precipitate: $2\text{AgNO}_3 + \text{Na}_2\text{CO}_3 \longrightarrow \text{Ag}_2\text{CO}_3 + 2\text{NaNO}_3$. Which of the compounds is the precipitate?

• A precipitate is an insoluble solid formed during a chemical reaction in solution.
• We need to identify which product is insoluble.
• According to solubility rules:
  • Nitrates ($\text{NO}_3^-$) are generally soluble. Thus, $\text{NaNO}_3$ is soluble.
  • Carbonates ($\text{CO}_3^{2-}$) are generally insoluble, with exceptions for Group 1 metals and ammonium. Silver ($\text{Ag}^+$) is not a Group 1 metal.
• Therefore, $\text{Ag}_2\text{CO}_3$ is the insoluble precipitate.

The correct answer is C. $\text{Ag}_2\text{CO}_3$.

5. Which of the properties of pure liquid varies with the amount of matter?

• Properties can be intensive (independent of amount) or extensive (dependent on amount).
• Boiling point, melting point, and density are intensive properties; they do not change with the amount of substance.
• Volume is an extensive property; it changes with the amount of substance. For example, 10 mL of water has a different volume than 100 mL of water.

The correct answer is D. Volume.

6. How many unpaired electrons are in a $\text{Mn}^{2+}$ ion in the ground electron state? [Mn = 25]

• First, write the electron configuration for neutral Manganese (Mn), which has 25 electrons: $$\text{Mn}: [\text{Ar}] 3d^5 4s^2$$
• To form the $\text{Mn}^{2+}$ ion, two electrons are removed. These are removed from the outermost shell first, which is the 4s orbital: $$\text{Mn}^{2+}: [\text{Ar}] 3d^5$$
• Now, consider the 3d subshell. It has 5 orbitals. According to Hund's rule, electrons will occupy each orbital singly before pairing up. $$3d^5: \uparrow \quad \uparrow \quad \uparrow \quad \uparrow \quad \uparrow$$
• All 5 electrons in the 3d subshell are unpaired.

The correct answer is D. 5.