Part II. Multiple choice, continued (3 points each). Name:__________________ Please circle your answer. There is only one correct answer to each question. 5. Which of the following molecular formulas is also an empirical formula? A) C6H6O2 B) C2H6SO C) H2O2 D) C6H6 6. What is the coefficient of O2 when the following equation is completed and balanced? C4H8O2 + O2 à ______________ A) 2 B) 3 C) 5 D) 6 7. Analysis of a compound shows that it contains 3.75 moles of C, 7.50 moles of H, and 1.25 moles of N. What is its empirical formula? A) CHN B) C3H6N C) C4H8N D) C5H9N2 8. What is the molar mass of Ca(NO3)2? (AW Ca = 40.1 amu; N = 14.0 amu; O = 16.0 amu) A) 70.1 g/mol B) 62.0 g/mol C) 164.1 g/mol D) 102.1 g/mol 9. Which of the following is an alkali metal? A) Mg B) Fe C) Al D) Na 10. A compound contains 40.0% C, 6.71% H, and 53.29% O by mass. The molecular weight of the compound is 60.05 amu. The molecular formula of this compound is A) C2H4O2 B) CH2O C) C2H3O4 D) C2H2O4 11. How many molecules of water, H2O, are there in 28.0 grams of water? A) 9.37 x 1023 B) 1.56 C) 578 D) 1.56 x 1025 12. When a hydrocarbon burns in air, what component of air reacts? A) nitrogen B) oxygen C) carbon dioxide D) water 13. When the following equation is balanced, the coefficient of Fe3O4 is__________. Al + Fe3O4 à Al2O3 + Fe A) 1 B) 2 C) 3 D) 4

Okay, let's dive into these chemistry concepts to help you understand the multiple-choice questions better.

5. Empirical Formula: The empirical formula represents the simplest whole number ratio of atoms in a compound. To determine if a molecular formula is also an empirical formula, check if the subscripts can be divided by a common number to get a simpler ratio.

   • A) C6H6O2 can be simplified to C3H3O.
   • B) C2H6SO cannot be simplified further.
   • C) H2O2 can be simplified to HO.
   • D) C6H6 can be simplified to CH.

   Therefore, the answer is B) C2H6SO because it is already in its simplest form.

6. Balancing Chemical Equations: To balance the equation C4H8O2 + O2 → CO2 + H2O, you need to ensure that the number of atoms for each element is the same on both sides of the equation.

   The balanced equation is:
   C4H8O2 + 5O2 → 4CO2 + 4H2O

   The coefficient of O2 is 5, so the answer is C) 5.

7. Determining Empirical Formula from Moles: To find the empirical formula, divide each mole value by the smallest number of moles present.

   • C: 3.75 moles
   • H: 7.50 moles
   • N: 1.25 moles

   Divide each by 1.25:

   • C: 3.75 / 1.25 = 3
   • H: 7.50 / 1.25 = 6
   • N: 1.25 / 1.25 = 1

   The empirical formula is C3H6N, so the answer is B) C3H6N.

8. Molar Mass Calculation: The molar mass of Ca(NO3)2 is calculated by adding the atomic weights of each element multiplied by the number of atoms of that element in the compound.

   • Ca: 40.1 amu
   • N: 14.0 amu (x2 = 28.0 amu)
   • O: 16.0 amu (x6 = 96.0 amu)

   Molar mass = 40.1 + 28.0 + 96.0 = 164.1 g/mol

   The answer is C) 164.1 g/mol.

9. Alkali Metals: Alkali metals are in Group 1 of the periodic table.

   • A) Mg (Magnesium) is an alkaline earth metal.
   • B) Fe (Iron) is a transition metal.
   • C) Al (Aluminum) is a post-transition metal.
   • D) Na (Sodium) is an alkali metal.

   The answer is D) Na.

10. Determining Molecular Formula from Percent Composition and Molecular Weight: First, find the empirical formula using the percentage composition, then determine the molecular formula using the molecular weight.

    Convert percentages to grams (assuming 100g):

    • C: 40.0 g
    • H: 6.71 g
    • O: 53.29 g

    Convert grams to moles:

    • C: 40.0 / 12.01 ≈ 3.33 moles
    • H: 6.71 / 1.008 ≈ 6.66 moles
    • O: 53.29 / 16.00 ≈ 3.33 moles

    Divide by the smallest number of moles (3.33):

    • C: 3.33 / 3.33 = 1
    • H: 6.66 / 3.33 = 2
    • O: 3.33 / 3.33 = 1

    Empirical formula: CH2O

    Calculate the empirical formula weight: 12.01 + 2(1.008) + 16.00 ≈ 30.03 amu

    Divide the molecular weight by the empirical formula weight: 60.05 / 30.03 ≈ 2

    Multiply the subscripts in the empirical formula by 2: C2H4O2

    The answer is A) C2H4O2.

11. Calculating Molecules from Grams: Use Avogadro's number (6.022 \times 10^{23}) to convert grams to molecules.

    First, calculate the number of moles of water:

    • Molar mass of water (H2O) = (2 \times 1.008) + 16.00 = 18.016 g/mol

    • Moles of water = 28.0 \text{ g} / 18.016 \text{ g/mol} ≈ 1.56 moles

    Now, convert moles to molecules:

    • 1.56 \text{ moles} \times 6.022 \times 10^{23} \text{ molecules/mole} ≈ 9.37 \times 10^{23} molecules

    The answer is A) 9.37 \times 10^{23}.

12. Combustion in Air: Combustion involves a substance reacting rapidly with oxygen to produce heat and light.

    The component of air that reacts in combustion is oxygen.

    The answer is B) oxygen.

13. Balancing Chemical Equations: Balance the equation Al + Fe3O4 → Al2O3 + Fe.

    The balanced equation is:

    8Al + 3Fe3O4 → 4Al2O3 + 9Fe

    The coefficient of Fe3O4 is 3, so the answer is C) 3.