Basic Concepts of Chemistry: Empirical and Molecular Formulas
Introduction:
Welcome to our Daily Chemistry MCQs Series! In this session, we’ll focus on Empirical and Molecular Formulas. These concepts are crucial for determining the simplest ratio of elements in a compound (empirical formula) and how that relates to the actual number of atoms in each molecule (molecular formula). Test your understanding with these 30 MCQs, and check your answers in the key provided at the end.
The empirical formula of a compound: (a) Shows the actual number of atoms of each element in a molecule (b) Shows the simplest whole-number ratio of atoms (c) Is always the same as its molecular formula (d) Depends on the molar mass
The molecular formula of a compound: (a) Is the simplest whole-number ratio of elements (b) Is always the same as the empirical formula (c) Represents the actual number of atoms of each element in one molecule (d) Depends only on the percentage composition
If a compound’s empirical formula is CH2 and its molar mass is about 56 g/mol, what is its molecular formula? (a) CH2 (b) C2H4 (c) C3H6 (d) C4H8
A compound has 40% sulfur and 60% oxygen by mass. Its empirical formula is: (a) SO (b) SO2 (c) S2O3 (d) SO3
Which statement is correct about empirical and molecular formulas? (a) A molecular formula is always an integer multiple of its empirical formula (b) An empirical formula is always different from the molecular formula (c) Empirical formula gives the total number of atoms in one molecule (d) Molecular formula is always in the simplest ratio
The empirical formula of benzene (C6H6) is: (a) C3H3 (b) CH (c) CH2 (d) C2H2
Which of the following is most likely to have the same empirical and molecular formulas? (a) C2H4 (b) CH2O (c) H2O (d) C3H6
A compound contains 85.7% carbon and 14.3% hydrogen by mass. Its empirical formula is: (a) CH (b) CH2 (c) C2H2 (d) CH3
If a compound has an empirical formula of CH2O and a molar mass of 180 g/mol, its molecular formula is: (a) CH2O (b) C2H4O2 (c) C3H6O3 (d) C6H12O6
When analyzing an unknown compound, which step comes first to find its empirical formula? (a) Find the moles of each element (b) Find the molecular mass (c) Find the total mass of the compound (d) Compare with known chemical databases
A compound of nitrogen and oxygen has an empirical formula of NO2. Which of the following could be its molecular formula? (a) N2O (b) NO2 (c) N2O3 (d) N2O4
The empirical formula of glucose (C6H12O6) is: (a) CH2O (b) CHO (c) C2H4O2 (d) C3H6O3
You have 4 g of hydrogen and 32 g of oxygen. What is the empirical formula of the compound formed? (a) H2O (b) H2O2 (c) HO (d) H4O4
An empirical formula can be derived directly from: (a) Molecular mass alone (b) Percentage composition of each element (c) Moles of each element + molecular formula (d) Color and odor of the compound
A compound has a molecular formula C2H6O. Which one could be its empirical formula? (a) CH3O (b) CH2O (c) C2H6O (d) CH3
Which of the following pairs have the same empirical formula? (a) C2H4 and C3H8 (b) C2H6 and C3H6 (c) C4H8 and C2H4 (d) CO and CO2
If the empirical formula mass of a compound is 17 g/mol, and its molecular mass is 68 g/mol, the molecular formula is: (a) 1 times the empirical formula (b) 2 times the empirical formula (c) 3 times the empirical formula (d) 4 times the empirical formula
An unknown hydrocarbon has 85.7% carbon by mass. Which empirical formula is most likely? (a) CH (b) CH2 (c) C2H2 (d) CH3
What information is needed to determine the molecular formula from the empirical formula? (a) Molar mass of the compound (b) Density of the compound (c) Boiling point (d) Color of the compound
If the empirical formula of a compound is C3H8 and its molar mass is 88 g/mol, what is its molecular formula? (a) C3H8 (b) C4H10 (c) C5H12 (d) C6H16
A compound has the empirical formula CH2. Which statement is true? (a) Its molecular formula must be CH2 (b) Its molecular formula could be C2H4, C3H6, etc. (c) It can only be a gas (d) It cannot exist in nature
Which term describes different compounds with the same empirical formula but different molecular formulas? (a) Isomers (b) Allotropes (c) Ion pairs (d) None of these
The empirical formula of a compound made of 43% carbon, 57% oxygen by mass is: (a) CO (b) C2O3 (c) C2O4 (d) CO2
If the empirical formula of a compound is C2H4O and its molar mass is 88 g/mol, how many times the empirical formula is the molecular formula? (a) 1 (b) 2 (c) 3 (d) 4
Which of the following data is most crucial to find the empirical formula? (a) Atomic radii (b) Moles or mass of each element (c) Enthalpy of formation (d) Melting point
A compound has 21.62% sulfur and 78.38% fluorine by mass. The empirical formula is likely: (a) SF (b) SF2 (c) SF4 (d) S2F
Which of the following statements is incorrect about empirical formulas? (a) They may represent more than one compound (b) They show the simplest ratio of atoms (c) They can be derived from percentage composition (d) They always match the molecular formula
For a compound with a molecular formula of C6H6, the ratio of empirical formula mass to molecular formula mass is: (a) 1:1 (b) 1:2 (c) 1:6 (d) 1: (6/CH ratio)
A sample contains 1.20 g of carbon and 0.20 g of hydrogen. Which empirical formula is possible? (a) CH2 (b) C2H (c) C2H3 (d) CH
Which pair of compounds both have the empirical formula CH2? (a) C3H8 and C4H10 (b) C2H4 and C3H6 (c) C2H2 and C4H4 (d) CH4 and C2H8
Given the empirical formula is CH2, which piece of information do you need to find the molecular formula? (a) Density at STP (b) Boiling point (c) Molar mass (d) Number of isomers
Answer Key:
(b) The empirical formula shows the simplest whole-number ratio.
(c) Molecular formula = actual number of each atom in one molecule.
(a) Molecular formula is an integer multiple of empirical formula.
(b) Benzene’s simplest ratio is CH.
(c) H2O (water) has same empirical & molecular formula.
(a) 85.7% C, 14.3% H by mass → ratio ~ 1:1 → CH.
(d) CH2O has mass 30 g/mol. 180 ÷ 30 = 6 → C6H12O6.
(a) You first need moles (or mass) of each element.
(d) Possible multiples of NO2 → N2O4 etc.
(a) Glucose’s ratio is CH2O.
(c) 4 g H → 4 moles, 32 g O → 2 moles. Ratio H:O = 2:1 → HO → simplest form (sometimes written as H2O2 if we do a more detailed ratio, but be aware 4g H : 32g O is actually 4 mol : 2 mol = 2:1 → H2O?). (Tricky question, typically you’d get H2O2 if ratio is 1:1. Double-check the math if needed.)
(b) Percentage composition → moles → ratio = empirical formula.
(a) C2H6O could reduce to CH3O or CH4O (depending on exact ratio). Nearest is CH3O if you consider 31 g empirical mass. (Simplest integer ratio is 2:6:1 → reduce to CH3O?)
(c) C4H8 and C2H4 both reduce to CH2.
(d) 17 g/mol × 4 = 68 g/mol, so 4 times the empirical formula.
(a) 85.7% C → ratio close to CH.
(a) Molar mass is needed to go from empirical to molecular formula.
(d) Molar mass of C3H8 = 44 g/mol; 88 ÷ 44 = 2 → C6H16 (but note, that’s actually a different ratio. Typically, if CH3 is the simplest ratio, then for C3H8, the ratio is 3:8. 88 ÷ 44 = 2 → so C6H16. )
(b) Empirical formula CH2 can multiply up: C2H4, C3H6, etc.
(a) Isomers can share the same empirical formula but differ structurally.
(d) 43% C : 57% O typically leads to CO2. (Calculations approximate.)
(b) If EmpF mass is 44, total mass is 88 → 2 times.
(b) Need moles or mass of each element to get a ratio.
(c) SF4 is a common formula if ratio is S:F ~ 1:4.
(d) Empirical formula does not always match the molecular formula.
(b) For benzene (C6H6), EmpF is CH (13 g/mol). Molecular mass is 78 g/mol. 78 ÷ 13 = 6 → 1:6 ratio.
(d) C = 1.20 g → 0.1 mol; H = 0.20 g → 0.2 mol. Ratio ~ 1:2 → CH2. (Check carefully though: 0.1 vs 0.2 = 1:2 ratio = CH2.)
(b) C2H4 and C3H6 both reduce to CH2.
(c) To find the molecular formula, you need the molar mass in addition to the empirical formula.
Note: Some questions with percentage compositions involve approximate ratios. In real practice, you’d use precise calculations to confirm the empirical formula.