Stoichiometry MCQs for MDCAT

Correct answer: D. 2 × 6.02 × 10²³ atoms

Two moles of chlorine atoms contain 2 × 6.02 × 10^23 atoms, applying Avogadro's number to the given number of moles.

Correct answer: B. Four

Since H2 and O2 react in a 2:1 ratio (2H2 + O2 to 2H2O), 4 moles of H2 fully reacting with the available O2 produces 4 moles of water.

Correct answer: A. 4.3 × 10⁻³

0.1 g of sodium (atomic mass ≈ 23 g/mol) corresponds to 0.1/23 ≈ 4.3×10^-3 moles.

Correct answer: B. Solution

Mole fraction is defined as the ratio of the moles of one component to the total moles of all components present in a solution.

Correct answer: B. Mole fraction of C₂H₅OH and H₂O is the same

Converting each component to moles (CH3OH ≈ 0.5 mol, C2H5OH = 2.0 mol, H2O = 2.0 mol) shows that the mole fractions of ethanol and water are equal, since they have the same number of moles.

Correct answer: C. Na2O

Let’s find the empirical formula step-by-step 👇1️⃣ Assume 100 g of the compound → Na = 74.2 g, O = 25.8 g2️⃣ Convert to moles: Na = 74.2 ÷ 23 = 3.23 mol O = 25.8 ÷ 16 = 1.61 mol3️⃣ Find the simplest ratio: Na : O = 3.23 ÷ 1.61 : 1.61 ÷ 1.61 = 2 : 1✅ Empirical formula = Na₂O

Correct answer: D. picric acid

Picric acid: Correct answer—When phenol is treated with concentrated nitric acid in the presence of sulfuric acid, it undergoes nitration, leading to the formation of picric acid (2,4,6-trinitrophenol), which is the major product.

Correct answer: D. Number of molecules are equal

The number of molecules is given by the formula:Number of molceules = moles x avogadros constantSince all of the given elements have the same number of moles all of them will have the same number of molecules, which will be 6.02 x 1023, the Avogadros constant.

Correct answer: A. Mass of an element

​Since molar mass is constant for a given element, increasing the mass increases the number of moles proportionally. For example, doubling the mass of an element doubles the number of moles, making them directly proportional.

Correct answer: D. Enthalpy of neutralization

The enthalpy of neutralization is the specific enthalpy change when one mole of water is formed from the reaction of an acid and a base (alkali) under standard conditions.

Correct answer: C. 18 moles of oxygen

Option C is correct. The balanced chemical equation is: 2 KClO3(l) + heat → 2 KCl(s) + 3 O2(g). This indicates that for every 2 moles of KClO3, 3 moles of O2 are produced. To find out how many moles of O2 are produced from 12 moles of KClO3, we set up the following calculation: 12 moles KClO3 * (3 moles O2 / 2 moles KClO3) = 18 moles O2. Therefore, the answer is 18 moles of oxygen. The other options are incorrect because they do not align with the stoichiometry of the reaction. Specifically, options A, B, and D propose amounts of O2 that exceed or do not reach the correct product ratio based on the given moles of KClO3.

Correct answer: C. 6.023x10 ^23

One mole of any substance contains 6.023 x 10^23 particles (atoms, molecules, or ions). This number is known as the Avogadro's number (NA).Avogadro's number is a fundamental constant in chemistry, and it is used to relate the amount of a substance to the number of particles it contains.