Question 1

1. An unknown element X has two naturally occurring isotopes. Isotope A has a mass of 62.93 u and Isotope B has a mass of 64.93 u. If the average atomic mass of X is 63.55 u, what is the percentage abundance of the lighter isotope?

A) 31.0%B) 50.0%C) 69.0%D) 75.5%

Answer

✓ Correct Answer: Option C

Explanation: Solution: Let the abundance of Isotope A (62.93 u) be x. Then abundance of Isotope B (64.93 u) is (1-x). $63.55 = 62.93(x) + 64.93(1-x) 63.55 = 62.93x + 64.93 - 64.93x 63.55 - 64.93 = -2x -1.38 = -2x ⇒ x = 0.69$ Percentage abundance = 69.0%.

Question 2

2. A mixture of 2.0 moles of $SO_2$ and 1.5 moles of $O_2$ is reacted in a closed vessel to form $SO_3$. After the reaction reaches completion, how many moles of total gas molecules are present in the vessel?

A) 2.0 moles

Question 3

3. An organic compound contains 40% Carbon, 6.67% Hydrogen, and the rest Oxygen. If the vapor density of the compound is 90, determine the molecular formula.

A) $C_6H_{12}O_6$B) $C_3H_6O_3$C) $CH_2O$D) $C_2H_4O_2$

Answer

✓ Correct Answer: Option A

Explanation: Carbon: 40/12 = 3.33. Hydrogen: 6.67/1 = 6.67. Oxygen: (100-46.67)/16 = 3.33. Ratio: C:H:O = 1:2:1. Empirical Formula = $CH_2O$. Empirical Formula Mass = 12 + 2 + 16 = 30. Molecular Mass = $2 \times Vapor Density = 2 \times 90 = 180$. n = 180/30 = 6. Molecular Formula = $6 \times (CH_2O) = C_6H_{12}O_6$.

Question 4

4. Calculate the molarity of a solution of ethanol in water in which the mole fraction of ethanol is 0.040 (assume the density of water to be 1 g/mL and the volume change on mixing is negligible).

A) 1.02 MB) 4.00 MC) 0.40 MD) 2.31 M

Answer

✓ Correct Answer: Option D

Explanation: Mole fraction $X_{et} = \frac{n_{et}}{n_{et} + n_w} = 0.040$. Let total moles be 1. $n_{et} = 0.04$ mol, $n_w = 0.96$ mol. Mass of water = $0.96 \times 18 = 17.28$ g. Volume of water (approx volume of solution) = 17.28 mL = 0.01728 L. Molarity = $\frac{0.04}{0.01728} \approx 2.31$ M.

Question 5

5. 10 g of $CaCO_3$ is treated with 100 mL of 0.5 M HCl. What is the volume of $CO_2$ liberated at STP?

A) 0.56 LB) 1.12 LC) 2.24 LD) 22.4 L

Answer

✓ Correct Answer: Option A

Explanation: Reaction: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$. Moles $CaCO_3 = 10/100 = 0.1$ mol. Moles $HCl = 0.5 \times 0.1 = 0.05$ mol. From reaction, 2 moles HCl react with 1 mole $CaCO_3$. 0.05 mol HCl will react with 0.025 mol $CaCO_3$. HCl is the limiting reagent. Moles of $CO_2$ formed = $1/2 \times moles of HCl = 0.025$ mol. Volume at STP = $0.025 \times 22.4 = 0.56$ L.

Question 6

6. The density of a 3.0 M solution of $Na_2S_2O_3$ is 1.25 g/mL. Calculate the molality of the solution. (Na=23, S=32, O=16)

A) 2.40 mB) 3.00 mC) 3.86 mD) 1.25 m

Answer

✓ Correct Answer: Option C

Explanation: Molar mass of $Na_2S_2O_3 = 2(23) + 2(32) + 3(16) = 158$ g/mol. In 1 L solution: Mass of solute = $3 \times 158 = 474$ g. Mass of solution = $1000 mL \times 1.25 g/mL = 1250$ g. Mass of solvent = 1250 - 474 = 776 g = 0.776 kg. Molality $m = \frac{moles solute}{mass solvent in kg} = \frac{3}{0.776} \approx 3.86$ m.

Question 7

7. Which of the following contains the same number of atoms as 12 g of Magnesium? (At. mass Mg=24, C=12, Ca=40)

A) 12 g of CarbonB) 20 g of CalciumC) 24 g of MagnesiumD) 1 mole of Helium

Answer

✓ Correct Answer: Option B

Explanation: Moles of Mg = 12/24 = 0.5 mol. Number of atoms = $0.5 \times N_A$. A) 12/12 = 1.0 mol. B) 20/40 = 0.5 mol. (Matches) C) 24/24 = 1.0 mol. D) 1.0 mol.

Question 8

8. A sample of $H_2SO_4$ is 98% by mass and has a density of 1.84 g/mL. What volume of this acid is required to make 5.0 L of 0.5 M $H_2SO_4$?

A) 125 mLB) 136 mLC) 250 mLD) 500 mL

Answer

✓ Correct Answer: Option B

Explanation: Molarity of stock $H_2SO_4 = \frac{% \times d \times 10}{M.W.} = \frac{98 \times 1.84 \times 10}{98} = 18.4$ M. Using $M_1V_1 = M_2V_2$: $18.4 \times V_1 = 0.5 \times 5000$ mL $V_1 = 2500 / 18.4 \approx 135.87$ mL \approx 136 mL.

Question 9

9. If 6.3 g of $NaHCO_3$ are added to 15.0 g of $CH_3COOH$ solution, the residue is found to weigh 18.0 g. What is the mass of $CO_2$ released in the reaction?

A) 4.2 gB) 1.5 gC) 2.1 gD) 3.3 g

Answer

✓ Correct Answer: Option D

Explanation: According to the Law of Conservation of Mass: Total mass of reactants = Total mass of products Mass of ($NaHCO_3 + CH_3COOH$) = Mass of (Residue + $CO_2$) $6.3 + 15.0 = 18.0 + mass of CO_2$ 21.3 = 18.0 + x x = 3.3 g.

Question 10

10. A gaseous hydrocarbon gives upon combustion 0.72 g of water and 3.08 g of $CO_2$. The empirical formula of the hydrocarbon is:

A) $CH_4$B) $C_2H_4$C) $C_7H_8$D) $C_3H_4$

Answer

✓ Correct Answer: Option C

Explanation: Moles of $CO_2 = 3.08 / 44 = 0.07$ mol. Moles of C = 0.07 mol. Moles of $H_2O = 0.72 / 18 = 0.04$ mol. Moles of $H = 2 \times 0.04 = 0.08$ mol. Ratio C:H = 0.07 : 0.08 = 7 : 8. Empirical Formula = $C_7H_8$.

Accepted Answer

✓ Correct Answer: Option B

Explanation: Reaction: $2SO_2 + O_2 \rightarrow 2SO_3$. Stoichiometric ratio of $SO_2 : O_2$ is 2:1. Given: 2.0 mol $SO_2$ and 1.5 mol $O_2$. $SO_2$ is the limiting reagent (requires 1.0 mol $O_2$). Moles of $O_2$ used = 1.0 mol; Excess $O_2 = 1.5 - 1.0 = 0.5$ mol. Moles of $SO_3$ formed = 2.0 mol. Total moles = Excess $O_2$ + Product $SO_3 = 0.5 + 2.0 = 2.5$ moles.

Hard Grade 11 Chemistry Quiz: Basic Concepts | Set 8

1. An unknown element X has two naturally occurring isotopes. Isotope A has a mass of 62.93 u and Isotope B has a mass of 64.93 u. If the average atomic mass of X is 63.55 u, what is the percentage abundance of the lighter isotope?

2. A mixture of 2.0 moles of $SO_2$ and 1.5 moles of $O_2$ is reacted in a closed vessel to form $SO_3$. After the reaction reaches completion, how many moles of total gas molecules are present in the vessel?

3. An organic compound contains 40% Carbon, 6.67% Hydrogen, and the rest Oxygen. If the vapor density of the compound is 90, determine the molecular formula.

4. Calculate the molarity of a solution of ethanol in water in which the mole fraction of ethanol is 0.040 (assume the density of water to be 1 g/mL and the volume change on mixing is negligible).

5. 10 g of $CaCO_3$ is treated with 100 mL of 0.5 M HCl. What is the volume of $CO_2$ liberated at STP?

6. The density of a 3.0 M solution of $Na_2S_2O_3$ is 1.25 g/mL. Calculate the molality of the solution. (Na=23, S=32, O=16)

7. Which of the following contains the same number of atoms as 12 g of Magnesium? (At. mass Mg=24, C=12, Ca=40)

8. A sample of $H_2SO_4$ is 98% by mass and has a density of 1.84 g/mL. What volume of this acid is required to make 5.0 L of 0.5 M $H_2SO_4$?

9. If 6.3 g of $NaHCO_3$ are added to 15.0 g of $CH_3COOH$ solution, the residue is found to weigh 18.0 g. What is the mass of $CO_2$ released in the reaction?

10. A gaseous hydrocarbon gives upon combustion 0.72 g of water and 3.08 g of $CO_2$. The empirical formula of the hydrocarbon is: