NEET Chemistry: Empirical and molecular formulae Practice Questions
9 free multiple-choice questions on Empirical and molecular formulae (Quantitative Analysis) for NEET Chemistry, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.
Practice thousands more NEET questions in the app →Q1. In a compound of Mg(OH)₂, the mass percentage of Magnesium is 24.3% and the mass percentage of Oxygen is 75.7%. What is the empirical formula of the compound?
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Correct answer: C — Mg(OH)₂
To find the empirical formula, we first find the atomic masses of Mg (24) and O (16). Mass of Mg = 24.3; Mass of O = 75.7. Relative number of moles of Mg = 24.3/24 = 1.0125. Relative number of moles of O = 75.7/16 = 4.73. The ratio 1.0125 : 4.73 is approximately 1:4, leading to the empirical formula Mg(OH)₂ (or Mg(OH) if simplifying the ratios based on the known formula).
Q2. Which of the following compounds has the empirical formula CH₂O?
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Correct answer: C — H₂O
For H₂O, the ratio of H to O is 2:1. Dividing by the smallest whole number gives 2:1. Thus, the empirical formula is H₂O.
Q3. The empirical formula of a compound of calcium nitrate (Ca(NO₃)₂) is Ca(NO₃)₂. If the molecular mass of the compound is 164 g/mol, what is the molecular formula?
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Correct answer: D — Ca(NO₃)₂
The molecular mass of Ca(NO₃)₂ is calculated as: Ca(40) + 2 × [N(14) + 3 × O(16)] = 40 + 2 × (14 + 48) = 40 + 2 × 62 = 164 g/mol. Since the calculated molecular mass matches the given molecular mass, the molecular formula is the same as the empirical formula, Ca(NO₃)₂.
Q4. If a compound contains 40% Carbon, 6.67% Hydrogen, and 53.33% Oxygen by mass, what is its empirical formula?
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Correct answer: C — CH2O
To find the empirical formula, assume a 100g sample. C = 40g/12 = 3.33; H = 6.67g/1 = 6.67; O = 53.33g/16.67 = 3.2. The simplest ratio is 3:1:2, leading to the empirical formula C3H2O. (In simplified terms, using the provided percentages directly leads to the ratio 3:1:2).
Q5. The empirical formula of a compound is obtained by dividing the mass of each element by its atomic mass and then finding the simplest whole-number ratio between these masses. This ratio is based on which fundamental concept?
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Correct answer: B — Law of Definite Proportions
The empirical formula is derived by taking the ratio of the masses of the constituent elements and reducing it to the simplest whole number ratio. This process relies on the Law of Definite Proportions, which states that a chemical compound always contains its constituent elements in a fixed ratio by mass, and Dalton's Atomic Theory regarding the relative atomic masses.
Q6. In a compound of Al₂O₃, the mass percentage of Aluminum is 80%. What is the empirical formula of the oxide?
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Correct answer: A — Al₂O₃
Since the compound is Al₂O₃, the ratio of Al to O is 2:3. This is the simplest whole number ratio, so the empirical formula is Al₂O₃.
Q7. The ratio of the masses of hydrogen and oxygen in a compound is 4:3. If the empirical formula of the compound is H₂O, what is the simplest whole number ratio of the atoms in the empirical formula?
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Correct answer: B — 1:1
The empirical formula represents the simplest whole number ratio of atoms. Since the ratio of H to O is already 4:3, and the empirical formula is H₂O, the ratio of atoms in the empirical formula is 2:1 for H and 1:1 for O (which simplifies the initial 4:3 ratio to the simplest integer form based on the empirical formula structure).
Q8. The molecular mass of water (H2O) is approximately 18.01 g/mol. If the empirical formula of a water-like compound is H₂O and its molecular mass is 36.03 g/mol, what is the relationship between the empirical formula mass and the molecular formula mass?
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Correct answer: C — The molecular mass is n times the empirical mass where n is the ratio of their subscripts
The molecular mass is obtained by multiplying the empirical formula mass by the integer n (which is the ratio of subscripts). For H₂O, the ratio is 36.03 / 18.01 ≈ 2.
Q9. In a compound of lead, sulfur, and oxygen, the mass of lead is 75%, the mass of sulfur is 25%, and the mass of oxygen is 0%, what is the simplest whole number ratio of the masses of lead, sulfur, and oxygen in the compound?
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Correct answer: A — 1:1:0
The mass percentages are Lead (75%), Sulfur (25%), and Oxygen (0%). The ratio of these masses is 75:25:0. Dividing by the greatest common divisor (25) gives the simplest whole number ratio: 3:1:0.
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