Elemental Composition of Pure Substances - AP Chemistry

1. Identify the primary element in the composition of ammonia (NH₃).

   Nitrogen. Nitrogen has the greatest atomic mass in NH₃ compared to the three hydrogens.

2. Determine the empirical formula of a compound with 79.9% carbon and 20.1% hydrogen.

   CH₃. Converting percentages to moles gives a 1:3 carbon to hydrogen ratio.

3. What is the chemical formula for a compound with 36.8% nitrogen and 63.2% oxygen?

   NO₂. Converting percentages to moles gives a 1:2 nitrogen to oxygen ratio.

4. Find the molar mass of glucose (C₆H₁₂O₆).

   180.$18 \text{ g/mol}$. Six carbons + twelve hydrogens + six oxygens: 72.06 + 12.12 + 96.00 = 180.18 g/mol.

5. What is the empirical formula of a compound with 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

   CH₂O. Convert percentages to moles, divide by smallest, giving 1:2:1 ratio.

6. Determine the empirical formula of a compound with 79.9% carbon and 20.1% hydrogen.

   CH₃. Converting percentages to moles gives a 1:3 carbon to hydrogen ratio.

7. Find the percent composition of chlorine in NaCl.

   60.66\text{ %}. Chlorine mass (35.45) divided by NaCl mass (58.44) times 100.

8. Find the molar mass of CO₂.

   44.$01 \text{ g/mol}$. Carbon (12.01) plus two oxygens (2 × 16.00) equals 44.01 g/mol.

9. What is the percent composition of carbon in $CO_2$?

   $27.29\%$ . Carbon mass (12.01) divided by $CO_2$ mass (44.01) times 100.

10. Find the percent composition of sulfur in Na₂SO₄.

    22.57\text{ %}. Sulfur mass (32.07) divided by Na₂SO₄ mass (142.05) times 100.

11. Determine the molar mass of ethanol (C₂H₅OH).

    46.$08 \text{ g/mol}$. Two carbons + six hydrogens + one oxygen: 24.02 + 6.06 + 16.00 = 46.08 g/mol.

12. Find the molar mass of CO₂.

    44.$01 \text{ g/mol}$. Carbon (12.01) plus two oxygens (2 × 16.00) equals 44.01 g/mol.

13. Determine the percent composition of hydrogen in water (H₂O).

    11.19\text{ %}. Two hydrogens (2.02 g) divided by water mass (18.02 g) times 100.

14. What is the empirical formula of a compound with 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

    CH₂O. Convert percentages to moles, divide by smallest, giving 1:2:1 ratio.

15. What is the percent composition of hydrogen in H₂SO₄?

    2.06\text{ %}. Two hydrogens (2.02 g) divided by total H₂SO₄ mass (98.08 g) times 100.

16. Calculate the percent composition of hydrogen in HCl.

    2.76\text{ %}. One hydrogen (1.01 g) divided by HCl mass (36.46 g) times 100.

17. What is the percent composition of oxygen in ethanol (C₂H₅OH)?

    34.73\text{ %}. One oxygen (16.00 g) divided by ethanol mass (46.08 g) times 100.

18. Find the molar mass of acetic acid (CH₃COOH).

    60.$05 \text{ g/mol}$. Two carbons + four hydrogens + two oxygens: 24.02 + 4.04 + 31.99 = 60.05 g/mol.

19. What is the chemical formula for a compound with 36.8% nitrogen and 63.2% oxygen?

    NO₂. Converting percentages to moles gives a 1:2 nitrogen to oxygen ratio.

20. Identify the primary element in the composition of ammonia (NH₃).

    Nitrogen. Nitrogen has the greatest atomic mass in NH₃ compared to the three hydrogens.

21. What is the molecular formula of a compound with an empirical formula of CH₂O and a molar mass of 180 g/mol?

    C₆H₁₂O₆. Molar mass 180 is six times the empirical formula mass of 30, so multiply by 6.

22. Find the molar mass of glucose (C₆H₁₂O₆).

    180.$18 \text{ g/mol}$. Six carbons + twelve hydrogens + six oxygens: 72.06 + 12.12 + 96.00 = 180.18 g/mol.

23. Determine the percent composition of oxygen in glucose (C₆H₁₂O₆).

    53.29\text{ %}. Six oxygens (96.00 g) divided by glucose mass (180.16 g) times 100.

24. Calculate the percent composition of oxygen in Na₂CO₃.

    45.28\text{ %}. Three oxygens (48.00 g) divided by Na₂CO₃ mass (105.99 g) times 100.

25. Find the molar mass of acetic acid (CH₃COOH).

    60.$05 \text{ g/mol}$. Two carbons + four hydrogens + two oxygens: 24.02 + 4.04 + 31.99 = 60.05 g/mol.

26. What is the empirical formula for a compound with 85.7% carbon and 14.3% hydrogen?

    CH₂. Converting percentages to moles gives a 1:2 carbon to hydrogen ratio.

27. What is the empirical formula for a compound with 85.7% carbon and 14.3% hydrogen?

    CH₂. Converting percentages to moles gives a 1:2 carbon to hydrogen ratio.

28. Calculate the percent composition of hydrogen in HCl.

    2.76\text{ %}. One hydrogen (1.01 g) divided by HCl mass (36.46 g) times 100.

29. Calculate the percent composition of calcium in CaCl₂.

    36.11\text{ %}. Calcium mass (40.08) divided by CaCl₂ mass (110.98 g) times 100.

30. Calculate the percent composition of hydrogen in HCl.

    2.76\text{ %}. One hydrogen (1.01 g) divided by HCl mass (36.46 g) times 100.

31. State the law that governs the fixed proportions by mass in chemical compounds.

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

32. Determine the molar mass of ethanol (C₂H₅OH).

    46.$08 \text{ g/mol}$. Two carbons + six hydrogens + one oxygen: 24.02 + 6.06 + 16.00 = 46.08 g/mol.

33. State the law that governs the fixed proportions by mass in chemical compounds.

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

34. Determine the percent composition of oxygen in glucose (C₆H₁₂O₆).

    53.29\text{ %}. Six oxygens (96.00 g) divided by glucose mass (180.16 g) times 100.

35. What is the formula for calculating percent composition by mass?

    $\% \text{ composition} = \frac{\text{mass of element}}{\text{total mass of compound}} \times 100$. Divides element mass by total compound mass, then multiplies by 100 for percentage.

36. What is the percent composition of hydrogen in acetic acid (C₂H₄O₂)?

    6.71\text{ %}. Four hydrogens (4.04 g) divided by acetic acid mass (60.05 g) times 100.

37. Which element has the highest percent composition in water (H₂O)?

    Oxygen. Oxygen has atomic mass 16, hydrogen has mass 1, so oxygen dominates in H₂O.

38. Calculate the percent composition of sulfur in H₂SO₄.

    32.69\text{ %}. Sulfur mass (32.07) divided by H₂SO₄ mass (98.08) times 100.

39. Identify the element with the highest composition in methane (CH₄).

    Carbon. Carbon has atomic mass 12.01, much greater than four hydrogens at 4.04 total.

40. Find the percent composition of chlorine in NaCl.

    60.66\text{ %}. Chlorine mass (35.45) divided by NaCl mass (58.44) times 100.

41. Calculate the percent composition of sulfur in H₂SO₄.

    32.69\text{ %}. Sulfur mass (32.07) divided by H₂SO₄ mass (98.08) times 100.

42. What is the molar mass of NaCl?

    58.$44 \text{ g/mol}$. Sodium (22.99) plus chlorine (35.45) equals 58.44 g/mol.

43. Which element has the highest percent composition in water (H₂O)?

    Oxygen. Oxygen has atomic mass 16, hydrogen has mass 1, so oxygen dominates in H₂O.

44. Determine the percent composition of hydrogen in water (H₂O).

    11.19\text{ %}. Two hydrogens (2.02 g) divided by water mass (18.02 g) times 100.

45. Calculate the percent composition of carbon in CH₄.

    74.87\text{ %}. Carbon mass (12.01) divided by methane mass (16.04) times 100.

46. What is the percent composition of oxygen in ethanol (C₂H₅OH)?

    34.73\text{ %}. One oxygen (16.00 g) divided by ethanol mass (46.08 g) times 100.

47. Find the percent composition of hydrogen in C₆H₁₂O₆.

    6.71\text{ %}. Twelve hydrogens (12.12 g) divided by glucose mass (180.16 g) times 100.

48. What law states the mass ratio between elements in a compound remains constant?

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

49. What is the formula for calculating percent composition by mass?

    $$. Divides element mass by total compound mass, then multiplies by 100 for percentage.

50. Find the molar mass of acetic acid (CH₃COOH).

    60.$05 \text{ g/mol}$. Two carbons + four hydrogens + two oxygens: 24.02 + 4.04 + 31.99 = 60.05 g/mol.

51. What is the molar mass of NaCl?

    58.$44 \text{ g/mol}$. Sodium (22.99) plus chlorine (35.45) equals 58.44 g/mol.

52. Calculate the molar mass of sulfuric acid (H₂SO₄).

    98.$08 \text{ g/mol}$. Two hydrogens + sulfur + four oxygens: 2.02 + 32.07 + 63.99 = 98.08 g/mol.

53. State the law that governs the fixed proportions by mass in chemical compounds.

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

54. What is the percent composition of oxygen in H₂O₂?

    94.06\text{ %}. Two oxygens (32.00 g) divided by H₂O₂ mass (34.02 g) times 100.

55. Determine the percent composition of hydrogen in water (H₂O).

    11.19\text{ %}. Two hydrogens (2.02 g) divided by water mass (18.02 g) times 100.

56. What is the empirical formula of a compound with 92.3% carbon and 7.7% hydrogen?

    CH. Converting percentages to moles gives a 1:1 carbon to hydrogen ratio.

57. What law states the mass ratio between elements in a compound remains constant?

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

58. What is the empirical formula for a compound with 62.1% carbon, 10.4% hydrogen, and 27.5% oxygen?

    C₃H₈O. Converting percentages to moles gives a 3:8:1 carbon to hydrogen to oxygen ratio.

59. Calculate the percent composition of sulfur in H₂SO₄.

    32.69\text{ %}. Sulfur mass (32.07) divided by H₂SO₄ mass (98.08) times 100.

60. Find the percent composition of sulfur in Na₂SO₄.

    22.57\text{ %}. Sulfur mass (32.07) divided by Na₂SO₄ mass (142.05) times 100.

61. What is the percent composition of oxygen in H₂O₂?

    94.06\text{ %}. Two oxygens (32.00 g) divided by H₂O₂ mass (34.02 g) times 100.

62. What is the empirical formula of a compound with 92.3% carbon and 7.7% hydrogen?

    CH. Converting percentages to moles gives a 1:1 carbon to hydrogen ratio.

63. Calculate the percent composition of carbon in CH₄.

    74.87\text{ %}. Carbon mass (12.01) divided by methane mass (16.04) times 100.

64. What is the molecular formula of a compound with an empirical formula of CH₂O and a molar mass of 180 g/mol?

    C₆H₁₂O₆. Molar mass 180 is six times the empirical formula mass of 30, so multiply by 6.

65. Find the percent composition of hydrogen in C₆H₁₂O₆.

    6.71\text{ %}. Twelve hydrogens (12.12 g) divided by glucose mass (180.16 g) times 100.

66. Determine the percent composition of oxygen in calcium carbonate (CaCO₃).

    48.00\text{ %}. Three oxygens (48.00 g) divided by CaCO₃ mass (100.09 g) times 100.

67. What is the percent composition of nitrogen in urea (CH₄N₂O)?

    46.65\text{ %}. Two nitrogens (28.02 g) divided by urea mass (60.06 g) times 100.

68. What is the percent composition of nitrogen in urea (CH₄N₂O)?

    46.65\text{ %}. Two nitrogens (28.02 g) divided by urea mass (60.06 g) times 100.

69. What is the empirical formula of a compound with 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

    CH₂O. Convert percentages to moles, divide by smallest, giving 1:2:1 ratio.

70. What is the percent composition of nitrogen in NH₄NO₃?

    35.00\text{ %}. Two nitrogens (28.02 g) divided by ammonium nitrate mass (80.04 g) times 100.

71. Identify the primary element in the composition of ammonia (NH₃).

    Nitrogen. Nitrogen has the greatest atomic mass in NH₃ compared to the three hydrogens.

72. What is the percent composition of oxygen in ethanol (C₂H₅OH)?

    34.73\text{ %}. One oxygen (16.00 g) divided by ethanol mass (46.08 g) times 100.

73. What is the percent composition of nitrogen in urea (CH₄N₂O)?

    46.65\text{ %}. Two nitrogens (28.02 g) divided by urea mass (60.06 g) times 100.

74. What law states the mass ratio between elements in a compound remains constant?

    Law of Definite Proportions. States that compounds always contain the same elements in fixed mass ratios.

75. What is the molecular formula of a compound with an empirical formula of CH₂O and a molar mass of 180 g/mol?

    C₆H₁₂O₆. Molar mass 180 is six times the empirical formula mass of 30, so multiply by 6.

76. Find the percent composition of chlorine in NaCl.

    60.66\text{ %}. Chlorine mass (35.45) divided by NaCl mass (58.44) times 100.

77. What is the empirical formula for a compound with 85.7% carbon and 14.3% hydrogen?

    CH₂. Converting percentages to moles gives a 1:2 carbon to hydrogen ratio.

78. Determine the percent composition of oxygen in calcium carbonate (CaCO₃).

    48.00\text{ %}. Three oxygens (48.00 g) divided by CaCO₃ mass (100.09 g) times 100.

79. What is the percent composition of oxygen in H₂O₂?

    94.06\text{ %}. Two oxygens (32.00 g) divided by H₂O₂ mass (34.02 g) times 100.

80. Find the percent composition of sulfur in Na₂SO₄.

    22.57\text{ %}. Sulfur mass (32.07) divided by Na₂SO₄ mass (142.05) times 100.

81. What is the empirical formula of a compound with 92.3% carbon and 7.7% hydrogen?

    CH. Converting percentages to moles gives a 1:1 carbon to hydrogen ratio.

82. What is the formula for calculating percent composition by mass?

    $\% \text{ composition} = \frac{\text{mass of element}}{\text{total mass of compound}} \times 100$. Divides element mass by total compound mass, then multiplies by 100 for percentage.

83. Determine the percent composition of oxygen in calcium carbonate (CaCO₃).

    48.00\text{ %}. Three oxygens (48.00 g) divided by CaCO₃ mass (100.09 g) times 100.

84. What is the percent composition of hydrogen in H₂SO₄?

    2.06\text{ %}. Two hydrogens (2.02 g) divided by total H₂SO₄ mass (98.08 g) times 100.

85. What is the percent composition of carbon in benzene (C₆H₆)?

    92.26\text{ %}. Six carbons (72.06 g) divided by benzene mass (78.12 g) times 100.

86. Which element has the highest percent composition in water (H₂O)?

    Oxygen. Oxygen has atomic mass 16, hydrogen has mass 1, so oxygen dominates in H₂O.

87. Calculate the percent composition of calcium in CaCl₂.

    36.11\text{ %}. Calcium mass (40.08) divided by CaCl₂ mass (110.98 g) times 100.

88. Calculate the percent composition of carbon in CH₄.

    74.87\text{ %}. Carbon mass (12.01) divided by methane mass (16.04) times 100.

89. Find the molar mass of CO₂.

    44.$01 \text{ g/mol}$. Carbon (12.01) plus two oxygens (2 × 16.00) equals 44.01 g/mol.

90. Calculate the percent composition of oxygen in Na₂CO₃.

    45.28\text{ %}. Three oxygens (48.00 g) divided by Na₂CO₃ mass (105.99 g) times 100.

91. What is the empirical formula for a compound with 62.1% carbon, 10.4% hydrogen, and 27.5% oxygen?

    C₃H₈O. Converting percentages to moles gives a 3:8:1 carbon to hydrogen to oxygen ratio.

92. Calculate the molar mass of sulfuric acid (H₂SO₄).

    98.$08 \text{ g/mol}$. Two hydrogens + sulfur + four oxygens: 2.02 + 32.07 + 63.99 = 98.08 g/mol.

93. Find the molar mass of glucose (C₆H₁₂O₆).

    180.$18 \text{ g/mol}$. Six carbons + twelve hydrogens + six oxygens: 72.06 + 12.12 + 96.00 = 180.18 g/mol.

94. Identify the element with the highest composition in methane (CH₄).

    Carbon. Carbon has atomic mass 12.01, much greater than four hydrogens at 4.04 total.

95. What is the percent composition of carbon in benzene (C₆H₆)?

    92.26\text{ %}. Six carbons (72.06 g) divided by benzene mass (78.12 g) times 100.

96. Determine the empirical formula of a compound with 79.9% carbon and 20.1% hydrogen.

    CH₃. Converting percentages to moles gives a 1:3 carbon to hydrogen ratio.

97. What is the percent composition of carbon in benzene (C₆H₆)?

    92.26\text{ %}. Six carbons (72.06 g) divided by benzene mass (78.12 g) times 100.

98. What is the percent composition of carbon in CO₂?

    27.29\text{ %}. Carbon mass (12.01) divided by CO₂ mass (44.01) times 100.

99. What is the percent composition of carbon in CO₂?

    27.29\text{ %}. Carbon mass (12.01) divided by CO₂ mass (44.01) times 100.

Study these flashcards