CALCULATIONS IN ORGANIC CHEMISTRY

(ORGANIC CHEMISTRY CALCULATION QUESTIONS AND ANSWERS)

 

The following questions and answers have been drawn randomly from organic chemistry. This should serve as revision guide for examinations. Be sure to input other variables (where necessary) for better understanding:

1. Question: Calculate the molecular weight of ethanol (C2H5OH).

   Answer: Molecular weight = (2 * 12.01) + (6 * 1.01) + 16.00 = 46.07 g/mol

 

2. Question: Determine the number of moles in 75 grams of glucose (C6H12O6).

   Answer: Moles = Mass / Molecular weight = 75 g / 180.16 g/mol ≈ 0.416 moles

 

3. Question: Calculate the percentage composition of carbon in methane (CH4).

   Answer: % Composition = (Mass of carbon / Molecular weight of CH4) * 100

   % Composition = (12.01 / 16.04) * 100 ≈ 74.96%

 

4. Question: How many hydrogen atoms are present in 2 moles of ammonia (NH3)?

   Answer: 2 moles NH3 * 3 hydrogen atoms per mole = 6 hydrogen atoms

 

5. Question: Calculate the molarity of a solution containing 0.5 moles of potassium chloride (KCl) in 250 mL of water.

   Answer: Molarity = Moles / Volume (in liters)

   Molarity = 0.5 moles / 0.25 L = 2.0 M

 

6. Question: What is the volume of 0.25 moles of methane gas (CH4) at STP (Standard Temperature and Pressure)?

   Answer: Volume = Moles * Molar volume at STP ≈ 0.25 moles * 22.7 L/mol ≈ 5.68 L

 

7. Question: Calculate the degree of unsaturation in C6H6 (benzene).

   Answer: Degree of unsaturation = (2 * (Number of carbons) + 2 - Number of hydrogens) / 2

   Degree of unsaturation = (2 * 6 + 2 - 6) / 2 = 3

 

8. Question: How many isomers are possible for C4H10O?

   Answer: Draw the structural formulas to find the isomers:

      a. Butanol (1-butanol)

      b. Isobutanol (2-methyl-1-propanol)

 

9. Question: Calculate the heat of combustion of octane (C8H18) given that the enthalpy change is -5471 kJ/mol.

   Answer: Heat of combustion = Enthalpy change * Number of moles

   Heat of combustion = -5471 kJ/mol * 1 mol = -5471 kJ

 

10. Question: How many stereoisomers are possible for 2,3-dibromobutane (C4H8Br2)?

    Answer: Draw the structural formulas to find the stereoisomers:

       a. (2R,3S)-2,3-dibromobutane

       b. (2S,3R)-2,3-dibromobutane

       c. (2S,3S)-2,3-dibromobutane

       d. (2R,3R)-2,3-dibromobutane

 

11. Question: Calculate the pH of a 0.01 M hydrochloric acid (HCl) solution.

    Answer: pH = -log[H+]

    pH = -log[0.01] ≈ 2

 

12. Question: What is the molecular formula of a compound with an empirical formula CH2O and a molar mass of 180.18 g/mol?

    Answer: Let the molecular formula be (CH2O)n, where n is an integer.

    n * (12.01 + 2 * 1.01 + 16.00) = 180.18

    n * 30.03 = 180.18

    n ≈ 6

    Molecular formula = (CH2O)6 = C6H12O6 (Glucose)

 

13. Question: Calculate the mass percent of nitrogen in ammonium sulfate (NH4)2SO4.

    Answer: Mass percent = (Mass of nitrogen / Molecular weight of (NH4)2SO4) * 100

    Mass percent = (2 * 14.01 / (2 * 14.01 + 4 * 1.01 + 32.06 + 4 * 16.00)) * 100 ≈ 21.21%

 

14. Question: How many sigma (σ) and pi (π) bonds are present in propene (C3H6)?

    Answer: Propene has 4 sigma bonds and 1 pi bond.

 

15. Question: Calculate the energy change when 50 grams of water vapor condense to liquid water at its boiling point (ΔHvap = 40.79 kJ/mol).

    Answer: Moles of water = Mass / Molar mass = 50 g / 18.02 g/mol ≈ 2.77 moles

    Energy change = Moles * ΔHvap = 2.77 moles * 40.79 kJ/mol ≈ 113.18 kJ

 

16. Question: What is the oxidation state of carbon in formaldehyde (CH2O)?

    Answer: Let the oxidation state of carbon be x.

    2 * 1 + x + 4 * (-2) = 0 (sum of oxidation states in a neutral compound)

    x - 8 = 0

    x = +2

 

17. Question: Calculate the percentage yield of a reaction where 25 grams of product were obtained, and the theoretical yield is 30 grams.

    Answer: Percentage yield = (Actual yield / Theoretical yield) * 100

    Percentage yield = (25 g / 30 g) * 100 ≈ 83.33%

 

18. Question: Determine the number of moles of oxygen gas (O2) in a 5.6 L container at 27°C and 2.0 atm.

    Answer: Moles = (Pressure * Volume) / (R * Temperature)

    Moles = (2.0 atm * 5.6 L) / (0.0821 atm·L/mol·K * (27°C + 273.15)) ≈ 0.475 moles

 

19. Question: Calculate the rate constant (k) of a first-order reaction with a half-life of 120 seconds.

    Answer: t1/2 = (0.693 / k)   (For first-order reactions)

    k = 0.693 / 120 s ≈ 0.00578 s^-1

 

20. Question: Calculate the molar absorptivity (ε) of a compound with an absorbance of 0.75, concentration of 0.10 M, and a path length of 1 cm.

    Answer: Absorbance (A) = ε * c * l

    ε = A / (c * l) = 0.75 / (0.10 M * 1 cm) = 7.5 L/(mol·cm)

 

21. Question: What is the pH of a 0.

 

02 M acetic acid (CH3COOH) solution (Ka = 1.8 x 10^-5)?

    Answer: Let x be the concentration of H+ ions from the dissociation.

    Ka = [H+][CH3COO-] / [CH3COOH]

    1.8 x 10^-5 = x^2 / (0.02 - x)   (Assuming x << 0.02)

    Solving for x, x ≈ 0.00424 M

    pH = -log[H+] = -log(0.00424) ≈ 2.37

 

22. Question: Calculate the vapor pressure of a solution containing 25 g of glucose (C6H12O6) in 500 mL of water at 25°C (vapor pressure of pure water at 25°C is 23.8 mmHg).

    Answer: Moles of glucose = Mass / Molar mass = 25 g / 180.16 g/mol ≈ 0.139 moles

    Moles of water = Volume (in liters) * Density / Molar mass = 0.5 L * 1 g/mL / 18.02 g/mol ≈ 0.027 moles

    Total moles of solute and solvent = 0.139 + 0.027 ≈ 0.166 moles

    Vapor pressure of solution = X(solvent) * Vapor pressure of pure solvent

    X(solvent) = Moles of solvent / Total moles = 0.027 / 0.166 ≈ 0.162

    Vapor pressure of solution = 0.162 * 23.8 mmHg ≈ 3.85 mmHg

 

23. Question: Determine the energy change (ΔE) for a reaction where 50 grams of methane (CH4) reacts with excess oxygen (ΔHcombustion = -802 kJ/mol).

    Answer: Moles of CH4 = Mass / Molar mass = 50 g / 16.04 g/mol ≈ 3.12 moles

    Energy change = Moles * ΔHcombustion = 3.12 moles * -802 kJ/mol ≈ -2500 kJ

 

24. Question: Calculate the oxidation state of sulfur in hydrogen sulfide (H2S).

    Answer: Let the oxidation state of sulfur be x.

    2 * 1 + x = 0   (sum of oxidation states in a neutral compound)

    x + 2 = 0

    x = -2

 

25. Question: How many structural isomers are possible for C4H10?

    Answer: Draw the structural formulas to find the isomers:

       a. Butane

       b. 2-Methylpropane (Isobutane)

 

26. Question: Calculate the concentration of a solution prepared by dissolving 15 grams of sodium chloride (NaCl) in 500 mL of water.

    Answer: Moles of NaCl = Mass / Molar mass = 15 g / 58.44 g/mol ≈ 0.257 moles

    Concentration = Moles / Volume (in liters) = 0.257 moles / 0.5 L = 0.514 M

 

27. Question: What volume of 1.5 M hydrochloric acid (HCl) is required to neutralize 25 mL of 2.0 M sodium hydroxide (NaOH)?

    Answer: Balanced equation: HCl + NaOH → NaCl + H2O

    Moles of NaOH = Concentration * Volume = 2.0 M * 0.025 L = 0.05 moles

    Moles of HCl required = 0.05 moles (from stoichiometry)

    Volume of HCl required = Moles / Concentration = 0.05 moles / 1.5 M ≈ 0.033 L ≈ 33 mL

 

28. Question: Calculate the degree of unsaturation in C6H12.

    Answer: Degree of unsaturation = (2 * (Number of carbons) + 2 - Number of hydrogens) / 2

    Degree of unsaturation = (2 * 6 + 2 - 12) / 2 = 1

 

29. Question: How many hydrogen atoms are present in 5 moles of ethylene (C2H4)?

    Answer: 5 moles C2H4 * 4 hydrogen atoms per mole = 20 hydrogen atoms

 

30. Question: Calculate the heat of combustion of methane (CH4) given that the enthalpy change is -890.4 kJ/mol.

    Answer: Heat of combustion = Enthalpy change * Number of moles

    Heat of combustion = -890.4 kJ/mol * 1 mol = -890.4 kJ

 

31. Question: Determine the number of moles of oxygen gas (O2) in a 10 L container at 300 K and 1.5 atm.

    Answer: Moles = (Pressure * Volume) / (R * Temperature)

    Moles = (1.5 atm * 10 L) / (0.0821 atm·L/mol·K * 300 K) ≈ 0.6 moles

 

32. Question: Calculate the pH of a 0.1 M hydrochloric acid (HCl) solution.

    Answer: pH = -log[H+]

    pH = -log(0.1) = 1

 

33. Question: What is the molecular formula of a compound with an empirical formula CH and a molar mass of 78.11 g/mol?

    Answer: Let the molecular formula be (CH)n, where n is an integer.

    n * (12.01 + 1.01) = 78.11

    n * 13.02 = 78.11

    n ≈ 6

    Molecular formula = (CH)6 = C6H6 (Benzene)

 

34. Question: Calculate the mass percent of nitrogen in ammonium nitrate (NH4NO3).

    Answer: Mass percent = (Mass of nitrogen / Molecular weight of NH4NO3) * 100

    Mass percent = 14.01 / (14.01 + 4 * 1.01 + 14.01 + 3 * 16.00) * 100 ≈ 35.03%

 

35. Question: How many sigma (σ) and pi (π) bonds are present in propyne (C3H4)?

    Answer: Propyne has 3 sigma bonds and 2 pi bonds.

 

36. Question: Calculate the energy change when 100 grams of water vapor condense to liquid water at its boiling point (ΔHvap = 40.79 kJ/mol).

    Answer: Moles of water = Mass / Molar mass = 100 g / 18.02 g/mol ≈ 5.55 moles

    Energy change = Moles * ΔHvap = 5.55 moles * 40.79 kJ/mol ≈ 226.23 kJ

 

37. Question: What is the oxidation state of carbon

 

 in carbon dioxide (CO2)?

    Answer: Let the oxidation state of carbon be x.

    2 * (-2) = x

    x = +4

 

38. Question: Calculate the percentage yield of a reaction where 40 grams of product were obtained, and the theoretical yield is 50 grams.

    Answer: Percentage yield = (Actual yield / Theoretical yield) * 100

    Percentage yield = (40 g / 50 g) * 100 ≈ 80%

 

39. Question: Determine the number of moles of chlorine gas (Cl2) in a 2 L container at 25°C and 1.5 atm.

    Answer: Moles = (Pressure * Volume) / (R * Temperature)

    Moles = (1.5 atm * 2 L) / (0.0821 atm·L/mol·K * (25°C + 273.15)) ≈ 0.123 moles

 

40. Question: Calculate the rate constant (k) of a second-order reaction with a half-life of 60 seconds.

    Answer: t1/2 = 1 / (k * [A]0)   (For second-order reactions)

    k = 1 / (t1/2 * [A]0) = 1 / (60 s * [A]0)

 

41. Question: Calculate the molar absorptivity (ε) of a compound with an absorbance of 0.5, concentration of 0.05 M, and a path length of 1 cm.

    Answer: Absorbance (A) = ε * c * l

    ε = A / (c * l) = 0.5 / (0.05 M * 1 cm) = 10 L/(mol·cm)

 

42. Question: What is the pH of a 0.005 M acetic acid (CH3COOH) solution (Ka = 1.8 x 10^-5)?

    Answer: Let x be the concentration of H+ ions from the dissociation.

    Ka = [H+][CH3COO-] / [CH3COOH]

    1.8 x 10^-5 = x^2 / (0.005 - x)   (Assuming x << 0.005)

    Solving for x, x ≈ 0.00424 M

    pH = -log[H+] = -log(0.00424) ≈ 2.37

 

43. Question: Calculate the vapor pressure of a solution containing 35 g of glucose (C6H12O6) in 500 mL of water at 25°C (vapor pressure of pure water at 25°C is 23.8 mmHg).

    Answer: Moles of glucose = Mass / Molar mass = 35 g / 180.16 g/mol ≈ 0.194 moles

    Moles of water = Volume (in liters) * Density / Molar mass = 0.5 L * 1 g/mL / 18.02 g/mol ≈ 0.027 moles

    Total moles of solute and solvent = 0.194 + 0.027 ≈ 0.221 moles

    Vapor pressure of solution = X(solvent) * Vapor pressure of pure solvent

    X(solvent) = Moles of solvent / Total moles = 0.027 / 0.221 ≈ 0.122

    Vapor pressure of solution = 0.122 * 23.8 mmHg ≈ 2.88 mmHg

 

44. Question: Determine the energy change (ΔE) for a reaction where 30 grams of ethane (C2H6) reacts with excess oxygen (ΔHcombustion = -1560 kJ/mol).

    Answer: Moles of C2H6 = Mass / Molar mass = 30 g / 30.07 g/mol ≈ 0.997 moles

    Energy change = Moles * ΔHcombustion = 0.997 moles * -1560 kJ/mol ≈ -1555 kJ

 

45. Question: Calculate the oxidation state of sulfur in sulfur dioxide (SO2).

    Answer: Let the oxidation state of sulfur be x.

    2 * (-2) = x

    x = +4

 

46. Question: How many structural isomers are possible for C5H12?

    Answer: Draw the structural formulas to find the isomers:

       a. Pentane

       b. 2-Methylbutane

       c. 2,2-Dimethylpropane (Neopentane)

 

47. Question: Calculate the concentration of a solution prepared by dissolving 10 grams of potassium permanganate (KMnO4) in 500 mL of water.

    Answer: Moles of KMnO4 = Mass / Molar mass = 10 g / 158.04 g/mol ≈ 0.063 moles

    Concentration = Moles / Volume (in liters) = 0.063 moles / 0.5 L = 0.126 M

 

48. Question: What volume of 2.0 M hydrochloric acid (HCl) is required to neutralize 30 mL of 1.5 M sodium hydroxide (NaOH)?

    Answer: Balanced equation: HCl + NaOH → NaCl + H2O

    Moles of NaOH = Concentration * Volume = 1.5 M * 0.03 L = 0.045 moles

    Moles of HCl required = 0.045 moles (from stoichiometry)

    Volume of HCl required = Moles / Concentration = 0.045 moles / 2.0 M ≈ 0.023 L ≈ 23 mL

 

49. Question: Calculate the degree of unsaturation in C7H8.

    Answer: Degree of unsaturation = (2 * (Number of carbons) + 2 - Number of hydrogens) / 2

    Degree of unsaturation = (2 * 7 + 2 - 8) / 2 = 6

 

50. Question: How many hydrogen atoms are present in 3 moles of propyne (C3H4)?

    Answer: 3 moles C3H4 * 4 hydrogen atoms per mole = 12 hydrogen atom.

 

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