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Quizzes > High School Quizzes > Science

Empirical & Molecular Formula Practice Quiz

Sharpen skills with formula problems and equations

Difficulty: Moderate
Grade: Grade 10
Study OutcomesCheat Sheet
Paper art representing a trivia quiz on chemical formulas for high school students.

What is an empirical formula?
It indicates the actual number of atoms in a molecule.
It shows the simplest whole number ratio of atoms in a compound.
It represents only the carbon content of an organic compound.
It provides the electron configuration of atoms in a compound.
An empirical formula represents the simplest whole number ratio of the elements in a compound. It is different from the molecular formula, which shows the actual number of atoms.
What is the empirical formula of H2O?
H2O2
H2O
H3O
HO
Water (H2O) is already in its simplest form with a 2:1 hydrogen to oxygen ratio. No further simplification is possible, so the empirical formula remains H2O.
What is the empirical formula of C2H6?
CH3
CH4
C2H6
CH2
Dividing the subscripts in C2H6 by the greatest common divisor (2) gives CH3. This represents the simplest whole number ratio for the compound.
Determine the empirical formula of a compound that is 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass.
CHO2
C2H4O2
CH2O
C2H5O
Assuming a 100 g sample, the masses convert to moles roughly as 3.33 for carbon, 6.7 for hydrogen, and 3.33 for oxygen. Dividing by the smallest value gives a ratio of 1:2:1, resulting in the empirical formula CH2O.
Which statement best defines a molecular formula?
It indicates the number of protons in the nucleus of each atom.
It shows the actual number of atoms of each element in a molecule.
It represents the simplest ratio of elements in a compound.
It is only used for ionic compounds.
The molecular formula tells you the exact number of atoms of each element in a molecule. This differentiates it from the empirical formula, which only provides the simplest ratio.
A compound has an empirical formula of CH2O and a molar mass of about 180 g/mol. What is its molecular formula?
C12H24O12
CH2O
C6H12O6
C3H6O3
The molar mass of the empirical formula CH2O is approximately 30 g/mol. Dividing 180 g/mol by 30 g/mol gives 6, so the molecular formula is 6 times the empirical formula: C6H12O6.
What is the percent composition of oxygen in carbon dioxide (CO2)?
~36.4%
~27.3%
~50.0%
~72.7%
CO2 has a molar mass of about 44 g/mol, with oxygen contributing 32 g/mol. Dividing 32 by 44 yields approximately 0.727, or 72.7% oxygen by mass.
How many moles of atoms are present in 1 mole of CH4?
1 mole
5 moles
6 moles
4 moles
A molecule of methane (CH4) contains 1 carbon atom and 4 hydrogen atoms, totaling 5 atoms. Therefore, 1 mole of CH4 contains 5 moles of atoms.
When determining an empirical formula from percent composition data, what is the first step?
Calculate the molecular weight of each element.
Write the formula based on valence electron counts.
Assume a 100 g sample to convert percentages to grams.
Determine the total number of atoms present.
Assuming a 100 g sample allows the percentages to be directly treated as grams. This simplifies the conversion from mass to moles, which is essential for finding the empirical formula.
A mineral is found to be 70% iron and 30% oxygen by mass. What is the empirical formula of this compound?
Fe2O3
FeO
Fe3O4
FeO2
Converting the percentages to moles and finding the simplest ratio yields approximately 1 mole of Fe for every 1.5 moles of O. Multiplying the ratio by 2 gives Fe2O3 as the empirical formula.
How do you determine the molecular formula when you already know the empirical formula and the molar mass?
Add the atomic masses of the empirical formula elements.
Multiply the empirical formula by the molar mass.
Divide the molar mass by the empirical formula mass and multiply each subscript by the result.
Divide the empirical formula by the molar mass.
To obtain the molecular formula, first calculate the mass of the empirical formula. Then, dividing the actual molar mass by this value provides a multiplier that, when applied to the empirical formula, gives the molecular formula.
Which of the following compounds has the same empirical formula as C6H12O6?
C3H6O3
CH2O
C2H4O2
C4H8O4
C6H12O6 simplifies by dividing each subscript by 6, which results in CH2O. This means that compounds with an empirical formula of CH2O share the same simplest ratio.
A compound is composed of 2.4 g of carbon, 0.4 g of hydrogen, and 3.2 g of oxygen. What is its empirical formula?
CH2O
C2H2O
C2H4O2
C3H6O
Converting the given masses to moles yields approximately 0.2 mol for carbon, 0.4 mol for hydrogen, and 0.2 mol for oxygen. The simplest ratio is 1:2:1, leading to the empirical formula CH2O.
If the molecular formula of a compound is C8H18, what is its empirical formula?
CH4
C2H5
C8H18
C4H9
Dividing the subscripts in C8H18 by their greatest common divisor (2) gives C4H9. This ratio is the simplest representation for the compound.
Which procedure is essential for determining the empirical formula from combustion analysis data?
Measure the boiling point of the combustion gases.
Calculate moles of CO2 and H2O produced to determine the amounts of C and H.
Record the volume of oxygen consumed during combustion.
Determine the color change during the reaction.
In combustion analysis, the amounts of CO2 and H2O produced are used to calculate the moles of carbon and hydrogen in the original compound. This data is crucial in deriving the empirical formula.
A 100 g sample of a calcium compound contains 40.08 g of Ca, 12.01 g of C, and 48.0 g of O. What is the empirical formula of the compound?
CaC2O6
CaCO3
CaCO2
Ca2CO3
Calculating the moles: Ca is 40.08/40.08 = 1, C is 12.01/12.01 = 1, and O is 48.0/16 = 3. This 1:1:3 ratio confirms the empirical formula CaCO3, which is calcium carbonate.
Determine the molecular formula of a compound if its empirical formula is C3H5N and its molar mass is approximately 110 g/mol.
C6H10N2
C9H15N3
C3H5N
C12H20N4
The mass of the empirical formula C3H5N is about 55 g/mol. Dividing the molecular mass (110 g/mol) by 55 g/mol gives a factor of 2; multiplying each subscript by 2 results in the molecular formula C6H10N2.
An unknown hydrocarbon weighing 5.0 g undergoes complete combustion and produces 13.8 g of CO2 and 11.3 g of H2O. What is its empirical formula?
CH4
C2H4
CH3
C2H8
From the combustion data, moles of CO2 (13.8 g ÷ 44 g/mol) yield approximately 0.314 mol of carbon, and moles of H2O (11.3 g ÷ 18 g/mol) yield about 0.628 mol of water, which gives 1.256 mol of hydrogen atoms. Dividing the moles of C and H by 0.314 results in a C:H ratio of roughly 1:4, corresponding to CH4.
Given the compound with molecular formula C10H12O, what is its degree of unsaturation?
4
5
6
3
The degree of unsaturation is calculated using the formula (2C + 2 - H)/2. For C10H12O, this calculation yields (20 + 2 - 12)/2 = 5, indicating five rings and/or double bonds in the structure.
An unknown hydrocarbon has an empirical formula of CH and a molecular mass of 78 g/mol. What is its molecular formula?
C7H7
CH
C12H12
C6H6
The empirical formula CH has a mass of 13 g/mol. Dividing the molecular mass (78 g/mol) by 13 g/mol gives 6, meaning the molecular formula is 6 times the empirical formula: C6H6.
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Study Outcomes

  1. Analyze experimental data to determine empirical formulas.
  2. Calculate molecular formulas using empirical data and molecular mass.
  3. Interpret chemical formulas to deduce molecular structures.
  4. Apply stoichiometric concepts to solve problems related to compound composition.
  5. Evaluate the relationship between empirical and molecular formulas in various compounds.

Empirical & Molecular Formula Practice Cheat Sheet

  1. Differentiate Empirical vs. Molecular Formulas - Chemistry loves precision! The empirical formula gives the simplest whole”number ratio of elements in a compound, while the molecular formula reveals the exact count of each atom in a single molecule. Mastering this distinction prevents calculation mix”ups and sharpens your problem”solving skills. Empirical vs. Molecular Formulas (OpenStax)
  2. Calculate Percent Composition - Percent composition breaks down a compound's mass into contributions from each element. By dividing each element's mass by the total mass and multiplying by 100%, you'll know exactly how much of each element is present. This skill is key for formula derivations and forensic analyses! Percent Composition Guide (OpenStax)
  3. Derive Empirical Formulas from Percent Data - Turn percentages into grams, convert grams into moles, and then simplify the ratios to the smallest whole numbers. These three steps form the recipe for uncovering a compound's simplest formula. Regular practice makes the process feel like second nature. Empirical Formula Practice (OpenStax)
  4. Determine Molecular Formulas - First get the empirical formula, then calculate its molar mass. Divide the compound's known molar mass by that empirical mass to find your multiplication factor. Multiply through to reveal the true molecular formula! Molecular Formula Steps (OpenStax)
  5. Tackle Practice Problems - Nothing beats hands”on experience. Work through varied exercises to reinforce both empirical and molecular formula concepts. Over time, you'll start spotting patterns and shortcuts that make these calculations a breeze. Practice Problems (LibreTexts)
  6. Leverage Online Practice Resources - Explore interactive quizzes, virtual labs, and step”by”step tutorials to sharpen your skills. Mixing up resources keeps studying fresh and uncovers new problem”solving strategies. Molecular Formula Practice (Pearson)
  7. Empirical Formulas for Ionic Compounds - Ionic solids don't exist as discrete molecules, so we always use their simplest ratios to represent them. Recognizing this pattern saves you from overcomplicating ionic compound formulas. Empirical Formula (Wikipedia)
  8. Use Mnemonic Devices - Memory tricks make multi”step calculations stick. Try "Percent to mass, mass to mole, divide by small, multiply 'til whole" and watch your formula derivations flow naturally. Mnemonic Devices (Wikipedia)
  9. Review Molar Mass Conversions - Converting between grams and moles underpins every formula calculation. Keep your periodic table handy and double”check atomic masses to avoid little arithmetic errors. Molar Mass Refresher (OpenStax)
  10. Stay Positive and Practice Regularly - Consistent, focused practice builds confidence and mastery. Celebrate small wins, tackle challenging problems, and remember that each attempt brings you closer to chemistry genius status!
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