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CHEM-1050 Test 3 Study Guidance: Chapters 7, 8, and 9

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Q1. Calculate the molar mass of magnesium chloride, MgCl2.

Background

Topic: Molar Mass Calculation

This question tests your ability to determine the molar mass of an ionic compound by summing the atomic masses of its constituent elements.

Key Terms and Formulas:

  • Molar mass: The mass (in grams) of one mole of a substance, calculated by adding the atomic masses of all atoms in the formula.

  • Atomic mass: The mass of a single atom, typically found on the periodic table (Mg ≈ 24.3 g/mol, Cl ≈ 35.5 g/mol).

Step-by-Step Guidance

  1. Identify the number of each type of atom in MgCl2: 1 magnesium (Mg) and 2 chlorine (Cl) atoms.

  2. Find the atomic masses for Mg and Cl from the periodic table.

  3. Multiply the atomic mass of Cl by 2 (since there are two Cl atoms).

  4. Add the mass of Mg to the total mass of Cl to get the molar mass of MgCl2.

Try solving on your own before revealing the answer!

Q2. What is the molar mass, to three significant figures, of calcium phosphate, Ca3(PO4)2?

Background

Topic: Molar Mass Calculation for Polyatomic Compounds

This question tests your ability to calculate the molar mass of a compound containing polyatomic ions.

Key Terms and Formulas:

  • Polyatomic ion: A charged group of covalently bonded atoms (e.g., PO43-).

  • Molar mass formula: Add the atomic masses of all atoms in the formula, accounting for subscripts and coefficients.

Step-by-Step Guidance

  1. Identify the number of each atom in Ca3(PO4)2: 3 Ca, 2 P, and 8 O.

  2. Find the atomic masses for Ca, P, and O from the periodic table.

  3. Multiply each atomic mass by the number of atoms present.

  4. Add all the masses together to get the total molar mass, rounding to three significant figures.

Try solving on your own before revealing the answer!

Q3. What is the percent by mass of carbon in methane, CH4?

Background

Topic: Percent Composition by Mass

This question tests your ability to calculate the percent by mass of a specific element in a compound.

Key Terms and Formulas:

  • Percent by mass:

  • Molar mass: Total mass of all atoms in the compound.

Step-by-Step Guidance

  1. Calculate the molar mass of CH4 by adding the atomic masses of 1 C and 4 H atoms.

  2. Find the mass of carbon in one mole of CH4 (just the atomic mass of C).

  3. Divide the mass of carbon by the molar mass of CH4 and multiply by 100% to get the percent by mass.

Try solving on your own before revealing the answer!

Q4. Calculate the empirical formula of a compound that has a composition of 5.9% hydrogen and 94.1% oxygen.

Background

Topic: Empirical Formula Determination

This question tests your ability to determine the simplest whole-number ratio of elements in a compound based on percent composition.

Key Terms and Formulas:

  • Empirical formula: The simplest whole-number ratio of atoms in a compound.

  • Steps: Convert percent to grams (assume 100 g sample), convert grams to moles, divide by the smallest number of moles, and round to nearest whole number.

Step-by-Step Guidance

  1. Assume a 100 g sample: 5.9 g H and 94.1 g O.

  2. Convert grams of H and O to moles using their atomic masses (H ≈ 1.01 g/mol, O ≈ 16.00 g/mol).

  3. Divide both mole values by the smallest number of moles to get the simplest ratio.

  4. Round the ratios to the nearest whole number to write the empirical formula.

Try solving on your own before revealing the answer!

Q5. Calculate the mass of the compound between calcium and sulfate that has the given composition: 29.4% calcium and 70.6% sulfate.

Background

Topic: Mass Percent and Compound Identification

This question tests your ability to use percent composition to determine the mass of a compound or to identify the compound based on its composition.

Key Terms and Formulas:

  • Percent composition: The percentage by mass of each element or group in a compound.

  • Mass calculation:

Step-by-Step Guidance

  1. Let the mass of the compound be x grams.

  2. Set up the equation: of x is calcium, of x is sulfate.

  3. Express the mass of calcium as and the mass of sulfate as .

  4. Use the given percentages to solve for x if a specific mass is provided, or use the ratio to identify the compound.

Try solving on your own before revealing the answer!

Q6. Which of the following gives the balanced equation for this reaction?

Background

Topic: Balancing Chemical Equations

This question tests your ability to recognize a correctly balanced chemical equation from a list of options.

Key Terms and Formulas:

  • Balanced equation: An equation where the number of atoms of each element is the same on both sides.

  • Law of Conservation of Mass: Matter is neither created nor destroyed in a chemical reaction.

Step-by-Step Guidance

  1. Write out the chemical formulas for all reactants and products.

  2. Count the number of atoms of each element on both sides of the equation.

  3. Adjust coefficients as needed to balance each element, starting with the most complex molecule.

  4. Check that all elements are balanced and that coefficients are in the lowest possible ratio.

Try solving on your own before revealing the answer!

Q7. What coefficient is placed in front of O2 to complete the balancing of the following equation? C3H8 (g) + O2 (g) → 5CO2 (g) + 4H2O (g)

Background

Topic: Balancing Combustion Reactions

This question tests your ability to balance a combustion reaction by determining the correct coefficient for oxygen gas.

Key Terms and Formulas:

  • Combustion reaction: A reaction where a hydrocarbon reacts with O2 to produce CO2 and H2O.

  • Balancing: Ensure the number of each type of atom is equal on both sides.

Step-by-Step Guidance

  1. Count the number of C, H, and O atoms on both sides of the equation.

  2. Balance C and H first, then balance O by adjusting the coefficient in front of O2.

  3. Remember that each O2 molecule provides 2 oxygen atoms.

  4. Set up an equation to solve for the O2 coefficient based on the total number of oxygen atoms needed on the product side.

Try solving on your own before revealing the answer!

Q9. What is the coefficient of hydrogen, H2, when the following equation is balanced? 2Al (s) + H2SO4 (aq) → Al2(SO4)3 (aq) + ? H2 (g)

Background

Topic: Balancing Single Replacement Reactions

This question tests your ability to balance a single replacement reaction and determine the correct coefficient for hydrogen gas.

Key Terms and Formulas:

  • Single replacement reaction: A reaction where one element replaces another in a compound.

  • Balancing: Make sure the number of each atom is the same on both sides.

Step-by-Step Guidance

  1. Write out the number of each atom on both sides of the equation.

  2. Balance Al and SO4 first, then balance H by adjusting the coefficient in front of H2.

  3. Remember that H2 is diatomic, so each molecule contains 2 H atoms.

  4. Check that all elements are balanced after assigning the coefficient to H2.

Try solving on your own before revealing the answer!

Q10. What is the coefficient for oxygen in the balanced equation for the combustion of pentane (C5H12)?

Background

Topic: Balancing Combustion Reactions

This question tests your ability to balance the combustion reaction of a hydrocarbon and determine the correct coefficient for O2.

Key Terms and Formulas:

  • Combustion reaction: Hydrocarbon + O2 → CO2 + H2O

  • Balancing: Balance C and H first, then O.

Step-by-Step Guidance

  1. Write the unbalanced equation: C5H12 + O2 → CO2 + H2O.

  2. Balance the number of C atoms by adjusting the coefficient in front of CO2.

  3. Balance the number of H atoms by adjusting the coefficient in front of H2O.

  4. Count the total number of O atoms needed on the product side and set up an equation to solve for the O2 coefficient.

Try solving on your own before revealing the answer!

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