Question 1

1. An analytical chemist examines two distinct oxides of a synthetic element E. In Oxide A, 1.00 \, g of E combines with 0.35 \, g of oxygen. In Oxide B, 1.00 \, g of E combines with 0.70 \, g of oxygen. Which chemical law is best illustrated by these findings?

A) Law of Conservation of MassB) Law of Definite ProportionsC) Law of Multiple ProportionsD) Avogadro's Law

Answer

✓ Correct Answer: Option C

Explanation: According to the Law of Multiple Proportions, if two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. Here, for a fixed mass (1.00 \, g) of E, the masses of oxygen are 0.35 \, g and 0.70 \, g. Ratio: $$\frac{0.70}{0.35} = \frac{2}{1}$$

Question 2

2. During a micro-gravity experiment, a spherical droplet of pure water is measured to have a mass of 90 \, mg. How many water molecules are contained within this single droplet? (Use $N_A = 6.022 \times 10^{23} \, mol^{-1}$)

Question 3

3. A specialized alloy used in aerospace contains 75% of metal M (Atomic mass = 50) and 25% of metal X (Atomic mass = 25) by mass. What is the empirical formula of this alloy?

A) $M_{1.5}X$ (or $M_3X_2$)B) $MX_2$C) $M_2X$D) MX

Answer

✓ Correct Answer: Option A

Explanation: 1. Relative moles of $M = \frac{75}{50} = 1.5$. 2. Relative moles of $X = \frac{25}{25} = 1.0$. 3. Simple ratio: M : X = 1.5 : 1 = 3 : 2. Empirical formula is $M_3X_2$.

Question 4

4. A technician needs to prepare 2.5 \, L of a 0.1 \, m solution of $KMnO_4$ for a titration. If the available stock solution is 0.5 \, m, what volume of the stock solution must be diluted?

A) 1.0 \, LB) 250 \, mLC) 125 \, mLD) 500 \, mL

Answer

✓ Correct Answer: Option D

Explanation: Using the dilution formula: $M_1V_1 = M_2V_2$. $0.5 \times V_1 = 0.1 \times 2.5$ $V_1 = \frac{0.25}{0.5} = 0.5 \, L = 500 \, mL$.

Question 5

5. Consider the synthesis of phosphine: $P_4(s) + 6H_2(g) \rightarrow 4PH_3(g)$. If 31 \, g of $P_4$ is reacted with 4 \, g of $H_2$, which reactant is the limiting reagent and how much $PH_3$ is formed? (Atomic mass: P=31, H=1)

A) $H_2$ is limiting; $34 \, g PH_3$B) $P_4$ is limiting; $68 \, g PH_3$C) $P_4$ is limiting; $34 \, g PH_3$D) $H_2$ is limiting; $22.6 \, g PH_3$

Answer

✓ Correct Answer: Option C

Explanation: 1. Moles of $P_4 = \frac{31}{124} = 0.25 \, mol$. 2. Moles of $H_2 = \frac{4}{2} = 2.0 \, mol$. 3. Required $H_2$ for $0.25 \, mol P_4 = 0.25 \times 6 = 1.5 \, mol$. Since we have $2.0 \, mol H_2$, $P_4$ is the limiting reagent. 4. Moles of $PH_3 = 4 \times 0.25 = 1 \, mol$. 5. Mass of $PH_3 = 1 \times (31 + 3) = 34 \, g$.

Question 6

6. A high-precision laser measures a distance as 0.002040 \, km. How many significant figures are present in this measurement?

A) 3B) 4C) 6D) 7

Answer

✓ Correct Answer: Option B

Explanation: Leading zeros are never significant. Trailing zeros after a decimal point are significant. The significant digits are 2, 0, 4, 0. Total = 4.

Question 7

7. An extraterrestrial rock sample contains element Q with two isotopes: $^{20}Q$ (90% abundance) and $^{22}Q$ (10% abundance). What is the average atomic mass of Q in this sample?

A) 20.2 \, uB) 21.0 \, uC) 20.5 \, uD) 21.8 \, u

Answer

✓ Correct Answer: Option A

Explanation: $$Average Atomic Mass = \sum (mass \times abundance) = (20 \times 0.90) + (22 \times 0.10) = 18.0 + 2.2 = 20.2 \, u$$

Question 8

8. What is the molality of a solution where 20 \, g of NaOH (Molar mass = 40 \, g/mol) is dissolved in 250 \, g of distilled water?

A) 1.0 \, mB) 0.5 \, mC) 4.0 \, mD) 2.0 \, m

Answer

✓ Correct Answer: Option D

Explanation: 1. Moles of $NaOH = \frac{20}{40} = 0.5 \, mol$. 2. Mass of solvent (water) in kg = 0.250 \, kg. 3. Molality (m) = $$\frac{moles of solute}{mass of solvent in kg} = \frac{0.5}{0.250} = 2.0 \, m$$

Question 9

9. A sample of 10 \, g of calcium carbonate ($CaCO_3$) is heated to decomposition: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$. What volume of $CO_2$ gas is released at STP? (Molar mass of $CaCO_3 = 100 \, g/mol$)

A) 22.4 \, LB) 11.2 \, LC) 2.24 \, LD) 4.48 \, L

Answer

✓ Correct Answer: Option C

Explanation: 1. Moles of $CaCO_3 = \frac{10}{100} = 0.1 \, mol$. 2. From the reaction, 1 mole of $CaCO_3$ gives 1 mole of $CO_2$. 3. Moles of $CO_2$ = 0.1 \, mol. 4. Volume at STP = $0.1 \times 22.4 \, L = 2.24 \, L$.

Question 10

10. Which of the following contains the largest number of atoms?

A) 1 \, g of Au(s)B) 1 \, g of Li(s)C) 1 \, g of Fe(s)D) 1 \, g of $O_2(g)$

Answer

✓ Correct Answer: Option B

Explanation: Number of atoms = $\frac{mass}{atomic mass} \times N_A$. For a fixed mass, the number of atoms is inversely proportional to the atomic mass. Atomic masses: Au \approx 197, Li \approx 7, Fe \approx 56, O \approx 16. Since Lithium (Li) has the lowest atomic mass among the options, 1 \, g of Li contains the most atoms. (Note: $O_2$ is diatomic, but the atomic mass of Li (7 \, u) is still much smaller than the atomic mass of O (16 \, u)).

Accepted Answer

✓ Correct Answer: Option B

Explanation: 1. Mass of water (m) = 90 \, mg = 0.090 \, g. 2. Molar mass of $H_2O$ (M) = 18 \, g/mol. 3. Moles (n) = $$\frac{m}{M} = \frac{0.090}{18} = 0.005 \, mol$ 4. Number of molecules =n \times N_A = 0.005 \times 6.022 \times 10^{23} = 3.011 \times 10^{21}$.

Grade 11 : Chemistry - Basic Concepts (Set 6)

1. An analytical chemist examines two distinct oxides of a synthetic element E. In Oxide A, 1.00 \, g of E combines with 0.35 \, g of oxygen. In Oxide B, 1.00 \, g of E combines with 0.70 \, g of oxygen. Which chemical law is best illustrated by these findings?

2. During a micro-gravity experiment, a spherical droplet of pure water is measured to have a mass of 90 \, mg. How many water molecules are contained within this single droplet? (Use $N_A = 6.022 \times 10^{23} \, mol^{-1}$)

3. A specialized alloy used in aerospace contains 75% of metal M (Atomic mass = 50) and 25% of metal X (Atomic mass = 25) by mass. What is the empirical formula of this alloy?

4. A technician needs to prepare 2.5 \, L of a 0.1 \, m solution of $KMnO_4$ for a titration. If the available stock solution is 0.5 \, m, what volume of the stock solution must be diluted?

5. Consider the synthesis of phosphine: $P_4(s) + 6H_2(g) \rightarrow 4PH_3(g)$. If 31 \, g of $P_4$ is reacted with 4 \, g of $H_2$, which reactant is the limiting reagent and how much $PH_3$ is formed? (Atomic mass: P=31, H=1)

6. A high-precision laser measures a distance as 0.002040 \, km. How many significant figures are present in this measurement?

7. An extraterrestrial rock sample contains element Q with two isotopes: $^{20}Q$ (90% abundance) and $^{22}Q$ (10% abundance). What is the average atomic mass of Q in this sample?

8. What is the molality of a solution where 20 \, g of NaOH (Molar mass = 40 \, g/mol) is dissolved in 250 \, g of distilled water?

9. A sample of 10 \, g of calcium carbonate ($CaCO_3$) is heated to decomposition: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$. What volume of $CO_2$ gas is released at STP? (Molar mass of $CaCO_3 = 100 \, g/mol$)

10. Which of the following contains the largest number of atoms?