Question 1

1. A robotic probe measures the density of a liquid on a distant moon to be $1.20 g/cm^3$. If the volume of the sample is recorded as $2.5 cm^3$, what is the mass of the sample reported with the correct number of significant figures?

A) 3 gB) 3.00 gC) 3.0 gD) 3.000 g

Answer

✓ Correct Answer: Option C

Explanation: In multiplication, the result must have the same number of significant figures as the measurement with the fewest significant figures. Density = 1.20 (3 sig figs) Volume = 2.5 (2 sig figs) $Mass = Density \times Volume = 1.20 \times 2.5 = 3.0 g$ The result must be reported to 2 significant figures.

Question 2

2. In a combustion analysis, a 0.45 g sample of a pure hydrocarbon produces 1.32 g of $CO_2$ and 0.81 g of $H_2O$. What is the empirical formula of the hydrocarbon?

A) $CH_2$

Question 3

3. A hypothetical reaction occurs: 2A + 5B \rightarrow 3C + 4D. If 0.4 moles of A and 0.8 moles of B are reacted, which reagent is limiting and how many moles of C are formed?

A) B is limiting; 0.48 mol CB) A is limiting; 0.60 mol CC) B is limiting; 0.80 mol CD) A is limiting; 0.40 mol C

Answer

✓ Correct Answer: Option A

Explanation: Required ratio B/A = 5/2 = 2.5. Given ratio B/A = 0.8/0.4 = 2.0. Since 2.0 < 2.5, B is the limiting reagent. Moles of C formed $= \frac{3}{5} \times moles of B = 0.6 \times 0.8 = 0.48 mol$.

Question 4

4. An isotope of a newly discovered element X has two naturally occurring isotopes: $^{70}X$ (abundance 60%) and $^{72}X$ (abundance 40%). What is the average atomic mass of element X?

A) 71.0 uB) 70.5 uC) 71.2 uD) 70.8 u

Answer

✓ Correct Answer: Option D

Explanation: Average atomic mass $= \sum (mass \times abundance)$ $= (70 \times 0.60) + (72 \times 0.40)$ = 42.0 + 28.8 = 70.8 u.

Question 5

5. Calculate the molarity of $Cl^-$ ions when 200 mL of $0.2 m MgCl_2$ solution is mixed with 300 mL of 0.1 m NaCl solution.

A) 0.14 MB) 0.22 MC) 0.18 MD) 0.25 M

Answer

✓ Correct Answer: Option B

Explanation: Moles of $Cl^-$ from $MgCl_2 = 2 \times (0.2 L \times 0.2 m) = 0.08 mol$ Moles of $Cl^-$ from $NaCl = 1 \times (0.3 L \times 0.1 m) = 0.03 mol$ Total moles of $Cl^- = 0.08 + 0.03 = 0.11 mol$ Total Volume = 0.2 + 0.3 = 0.5 L Molarity $= \frac{0.11}{0.5} = 0.22 m$.

Question 6

6. How many oxygen atoms are present in 88 g of $CO_2$? (Take $N_A = 6.022 \times 10^{23}$)

A) $1.204 \times 10^{24}$B) $6.022 \times 10^{23}$C) $2.408 \times 10^{24}$D) $3.011 \times 10^{23}$

Answer

✓ Correct Answer: Option C

Explanation: Moles of $CO_2 = \frac{88}{44} = 2 mol$. Each molecule of $CO_2$ has 2 Oxygen atoms. Total O atoms $= 2 \times 2 \times N_A = 4 \times 6.022 \times 10^{23} = 2.408 \times 10^{24}$ atoms.

Question 7

7. Which of the following pairs of compounds illustrates the Law of Multiple Proportions?

A) $SO_2$ and $SO_3$B) $H_2O$ and $D_2O$C) NaCl and NaBrD) MgO and $Mg(OH)_2$

Answer

✓ Correct Answer: Option A

Explanation: The Law of Multiple Proportions states that when two elements combine to 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. $SO_2$ and $SO_3$ consist of the same two elements (S and O) in different ratios.

Question 8

8. A solution is prepared by dissolving 10 g of a non-volatile solute in 81 g of water. The mole fraction of the solute is 0.1. What is the molar mass of the solute?

A) 18 g/molB) 40 g/molC) 60 g/molD) 20 g/mol

Answer

✓ Correct Answer: Option D

Explanation: $n_{water} = \frac{81}{18} = 4.5 mol$. Let $n_{solute}$ be x. Mole fraction $X_{solute} = \frac{x}{x + 4.5} = 0.1$ x = 0.1x + 0.45 \Rightarrow 0.9x = 0.45 \Rightarrow x = 0.5 mol. Molar mass $= \frac{Mass}{moles} = \frac{10}{0.5} = 20 g/mol$.

Question 9

9. What is the mass of 11.2 L of $N_2$ gas at STP?

A) 28 gB) 7 gC) 14 gD) 22.4 g

Answer

✓ Correct Answer: Option C

Explanation: 1 mole of any gas at STP occupies 22.4 L. Moles of $N_2 = \frac{11.2}{22.4} = 0.5 mol$. Molar mass of $N_2 = 28 g/mol$. Mass $= 0.5 \times 28 = 14 g$.

Question 10

10. In the reaction X + 2Y \rightarrow Z, if you start with 5 moles of X and 8 moles of Y, and the reaction has a 75% yield, how many moles of Z are actually produced?

A) 4 molB) 3 molC) 3.75 molD) 5 mol

Answer

✓ Correct Answer: Option B

Explanation: 1. Limiting Reagent: Y is limiting (8/2 = 4 vs 5/1 = 5). 2. Theoretical yield of $Z = \frac{1}{2} \times moles of Y = 4 moles$. 3. Actual yield $= 75% of 4 = 0.75 \times 4 = 3 moles$.

Accepted Answer

✓ Correct Answer: Option B

Explanation: 1. Moles of $C = \frac{1.32}{44.01} \approx 0.03 mol$ 2. Moles of $H = 2 \times \frac{0.81}{18.02} \approx 0.09 mol$ 3. Ratio C:H = 0.03 : 0.09 = 1:3 Empirical Formula = $CH_3$.

Grade 11 : Chemistry - Basic Concepts (Set 5)

1. A robotic probe measures the density of a liquid on a distant moon to be $1.20 g/cm^3$. If the volume of the sample is recorded as $2.5 cm^3$, what is the mass of the sample reported with the correct number of significant figures?

2. In a combustion analysis, a 0.45 g sample of a pure hydrocarbon produces 1.32 g of $CO_2$ and 0.81 g of $H_2O$. What is the empirical formula of the hydrocarbon?

3. A hypothetical reaction occurs: 2A + 5B \rightarrow 3C + 4D. If 0.4 moles of A and 0.8 moles of B are reacted, which reagent is limiting and how many moles of C are formed?

4. An isotope of a newly discovered element X has two naturally occurring isotopes: $^{70}X$ (abundance 60%) and $^{72}X$ (abundance 40%). What is the average atomic mass of element X?

5. Calculate the molarity of $Cl^-$ ions when 200 mL of $0.2 m MgCl_2$ solution is mixed with 300 mL of 0.1 m NaCl solution.

6. How many oxygen atoms are present in 88 g of $CO_2$? (Take $N_A = 6.022 \times 10^{23}$)

7. Which of the following pairs of compounds illustrates the Law of Multiple Proportions?

8. A solution is prepared by dissolving 10 g of a non-volatile solute in 81 g of water. The mole fraction of the solute is 0.1. What is the molar mass of the solute?

9. What is the mass of 11.2 L of $N_2$ gas at STP?

10. In the reaction X + 2Y \rightarrow Z, if you start with 5 moles of X and 8 moles of Y, and the reaction has a 75% yield, how many moles of Z are actually produced?