General Chemistry Multiple Choice Practice – Step-by-Step Guidance

Q4. Determine the percent carbon by mass in the acetate ion CH3COO-.

Background

Topic: Percent Composition by Mass

This question tests your ability to calculate the percent by mass of a specific element (carbon) in a polyatomic ion (acetate).

Key Terms and Formulas

• Percent by mass: The fraction of the total mass of a compound that is due to a particular element, expressed as a percentage.

• Molar mass: The sum of the atomic masses of all atoms in a formula.

Key formula:

$\text{Percent by mass of element} = \frac{\text{Total mass of element in formula}}{\text{Molar mass of compound}} \times 100\%$

Step-by-Step Guidance

1. Write the formula for the acetate ion: $\mathrm{CH_3COO^-}$ (or $\mathrm{C_2H_3O_2^-}$).

2. Calculate the molar mass of the acetate ion by adding the atomic masses of all atoms present (2 C, 3 H, 2 O).

3. Calculate the total mass of carbon in one acetate ion (2 atoms of C).

4. Set up the percent by mass formula using the values from steps 2 and 3, but do not calculate the final value yet.

Try solving on your own before revealing the answer!

Q5. Which compound is INCORRECTLY named?

Background

Topic: Nomenclature of Ionic Compounds

This question tests your understanding of how to correctly name ionic compounds, including those with polyatomic ions and transition metals.

Key Terms and Concepts

• Roman numerals indicate the oxidation state of transition metals.

• Polyatomic ions have specific names (e.g., sulfate, sulfite, nitrate, dichromate).

• Check for correct pairing of formula and name.

Step-by-Step Guidance

1. Review each formula and its corresponding name, focusing on the oxidation state and the correct name for the polyatomic ion.

2. For each, check if the chemical formula matches the name (e.g., Fe(OH)3 should be iron(III) hydroxide, not iron(II)).

3. Identify the one where the formula and the name do not match according to IUPAC rules.

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Q6. Determine the molecular mass of iron (II) bromide, a substance used as a catalyst in organic reactions.

Background

Topic: Molar Mass Calculation

This question tests your ability to calculate the molar (molecular) mass of an ionic compound using atomic masses from the periodic table.

Key Terms and Formulas

• Molecular (or formula) mass: The sum of the atomic masses of all atoms in the formula.

• Iron (II) bromide formula: $\mathrm{FeBr_2}$

Key formula:

$\text{Molecular mass} = (\text{atomic mass of Fe}) + 2 \times (\text{atomic mass of Br})$

Step-by-Step Guidance

1. Write the formula for iron (II) bromide: $\mathrm{FeBr_2}$.

2. Look up the atomic masses: Fe (about 55.85 amu), Br (about 79.90 amu).

3. Multiply the atomic mass of Br by 2, then add the atomic mass of Fe.

4. Set up the calculation, but do not compute the final value yet.

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Q7. How many moles of sodium ions are in a 100 mL of 0.1 M solution of Na3PO4?

Background

Topic: Moles, Molarity, and Dissociation of Ionic Compounds

This question tests your understanding of how to use molarity and volume to find moles, and how to account for the number of ions produced when a strong electrolyte dissociates.

Key Terms and Formulas

• Molarity (M): moles of solute per liter of solution.

• Volume must be in liters for calculations.

• Na3PO4 dissociates into 3 Na+ ions and 1 PO43- ion per formula unit.

Key formula:

$\text{Moles of solute} = \text{Molarity} \times \text{Volume (L)}$

Step-by-Step Guidance

1. Convert 100 mL to liters (divide by 1000).

2. Calculate the moles of Na3PO4 in the solution using the formula above.

3. Multiply the moles of Na3PO4 by 3 to get the moles of Na+ ions (since each formula unit gives 3 Na+).

4. Set up the calculation, but do not compute the final value yet.

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Q8. The distance between carbon atoms in ethylene is 134 picometers. Which of the following expresses that distance in meters?

Background

Topic: Metric Conversions (Scientific Notation)

This question tests your ability to convert between units (picometers to meters) and express the answer in scientific notation.

Key Terms and Formulas

• 1 picometer (pm) = $1 \times 10^{-12}$ meters (m)

• Scientific notation: expressing numbers as a coefficient times a power of ten.

Step-by-Step Guidance

1. Write the conversion factor: $1\ \text{pm} = 1 \times 10^{-12}\ \text{m}$.

2. Multiply 134 pm by the conversion factor to get the value in meters.

3. Express the result in proper scientific notation, but do not calculate the final value yet.

Try solving on your own before revealing the answer!