Praxis Chemistry Stoichiometry and Molar Mass Quiz

1. What is the molar mass of H₂SO₄ (sulfuric acid)?

98 g/mol

108 g/mol

118 g/mol

128 g/mol

To calculate the molar mass of H₂SO₄, sum the atomic masses of its constituent elements: hydrogen (H) has a mass of about 1 g/mol, sulfur (S) about 32 g/mol, and oxygen (O) about 16 g/mol. Thus, H₂SO₄ consists of 2(H) + 1(S) + 4(O) = 2 + 32 + 64 = 98 g/mol.

Explanation

To calculate the molar mass of H₂SO₄, sum the atomic masses of its constituent elements: hydrogen (H) has a mass of about 1 g/mol, sulfur (S) about 32 g/mol, and oxygen (O) about 16 g/mol. Thus, H₂SO₄ consists of 2(H) + 1(S) + 4(O) = 2 + 32 + 64 = 98 g/mol.

2. How many moles are in 54 grams of water (H₂O)?

2 mol

3 mol

4 mol

5 mol

To find the number of moles in 54 grams of water (H₂O), divide the mass by the molar mass of water, which is approximately 18 g/mol. Thus, 54 g ÷ 18 g/mol = 3 moles. This calculation shows that 54 grams of water contains 3 moles.

Explanation

To find the number of moles in 54 grams of water (H₂O), divide the mass by the molar mass of water, which is approximately 18 g/mol. Thus, 54 g ÷ 18 g/mol = 3 moles. This calculation shows that 54 grams of water contains 3 moles.

3. In the reaction 2H₂ + O₂ → 2H₂O, what is the mole ratio of H₂ to O₂?

1:1

2:1

1:2

2:2

In the given reaction, 2 moles of hydrogen gas (H₂) react with 1 mole of oxygen gas (O₂) to produce 2 moles of water (H₂O). This indicates that for every 2 moles of H₂, only 1 mole of O₂ is required, establishing a mole ratio of 2:1 for H₂ to O₂.

Explanation

In the given reaction, 2 moles of hydrogen gas (H₂) react with 1 mole of oxygen gas (O₂) to produce 2 moles of water (H₂O). This indicates that for every 2 moles of H₂, only 1 mole of O₂ is required, establishing a mole ratio of 2:1 for H₂ to O₂.

4. What is the mass of 0.5 moles of NaCl (sodium chloride)?

14.6 g

29.2 g

58.4 g

116.8 g

To find the mass of 0.5 moles of NaCl, first calculate the molar mass of NaCl, which is approximately 58.44 g/mol. Multiplying the molar mass by the number of moles (0.5) gives 29.22 g, which is rounded to 29.2 g. Thus, 0.5 moles of NaCl has a mass of 29.2 g.

Explanation

To find the mass of 0.5 moles of NaCl, first calculate the molar mass of NaCl, which is approximately 58.44 g/mol. Multiplying the molar mass by the number of moles (0.5) gives 29.22 g, which is rounded to 29.2 g. Thus, 0.5 moles of NaCl has a mass of 29.2 g.

5. How many grams of CO₂ are produced from 1 mole of C in the reaction C + O₂ → CO₂?

28 g

32 g

44 g

64 g

In the reaction C + O₂ → CO₂, one mole of carbon (C) reacts with one mole of oxygen (O₂) to produce one mole of carbon dioxide (CO₂). The molar mass of CO₂ is 12 g (for carbon) + 32 g (for oxygen) = 44 g. Thus, 1 mole of C produces 44 grams of CO₂.

Explanation

In the reaction C + O₂ → CO₂, one mole of carbon (C) reacts with one mole of oxygen (O₂) to produce one mole of carbon dioxide (CO₂). The molar mass of CO₂ is 12 g (for carbon) + 32 g (for oxygen) = 44 g. Thus, 1 mole of C produces 44 grams of CO₂.

6. What is the molar mass of Ca(OH)₂ (calcium hydroxide)?

54 g/mol

64 g/mol

74 g/mol

84 g/mol

To calculate the molar mass of Ca(OH)₂, add the atomic masses of calcium (Ca, 40 g/mol), oxygen (O, 16 g/mol, two atoms), and hydrogen (H, 1 g/mol, two atoms). The calculation is: 40 + (2 × 16) + (2 × 1) = 74 g/mol. Thus, the molar mass of calcium hydroxide is 74 g/mol.

Explanation

To calculate the molar mass of Ca(OH)₂, add the atomic masses of calcium (Ca, 40 g/mol), oxygen (O, 16 g/mol, two atoms), and hydrogen (H, 1 g/mol, two atoms). The calculation is: 40 + (2 × 16) + (2 × 1) = 74 g/mol. Thus, the molar mass of calcium hydroxide is 74 g/mol.

7. If you have 180 grams of glucose (C₆H₁₂O₆), how many moles do you have?

0.5 mol

1 mol

1.5 mol

2 mol

To find the number of moles of glucose, use the formula: moles = mass (g) / molar mass (g/mol). The molar mass of glucose (C₆H₁₂O₆) is approximately 180 g/mol. Therefore, 180 grams of glucose corresponds to 1 mole (180 g / 180 g/mol = 1 mol).

Explanation

To find the number of moles of glucose, use the formula: moles = mass (g) / molar mass (g/mol). The molar mass of glucose (C₆H₁₂O₆) is approximately 180 g/mol. Therefore, 180 grams of glucose corresponds to 1 mole (180 g / 180 g/mol = 1 mol).

8. In the balanced equation 4Fe + 3O₂ → 2Fe₂O₃, how many moles of Fe₂O₃ form from 4 moles of Fe?

1 mol

2 mol

3 mol

4 mol

In the balanced equation, 4 moles of iron (Fe) react with 3 moles of oxygen (O₂) to produce 2 moles of iron(III) oxide (Fe₂O₃). Therefore, from 4 moles of Fe, the stoichiometry indicates that 2 moles of Fe₂O₃ are formed, demonstrating the direct proportionality in the reaction.

Explanation

In the balanced equation, 4 moles of iron (Fe) react with 3 moles of oxygen (O₂) to produce 2 moles of iron(III) oxide (Fe₂O₃). Therefore, from 4 moles of Fe, the stoichiometry indicates that 2 moles of Fe₂O₃ are formed, demonstrating the direct proportionality in the reaction.

9. What is the molar mass of Al₂O₃ (aluminum oxide)?

78 g/mol

102 g/mol

126 g/mol

150 g/mol

To calculate the molar mass of Al₂O₃, add the atomic masses of its components: aluminum (Al) has an atomic mass of about 27 g/mol, and oxygen (O) about 16 g/mol. Thus, for Al₂O₃: (2 × 27) + (3 × 16) = 54 + 48 = 102 g/mol.

Explanation

To calculate the molar mass of Al₂O₃, add the atomic masses of its components: aluminum (Al) has an atomic mass of about 27 g/mol, and oxygen (O) about 16 g/mol. Thus, for Al₂O₃: (2 × 27) + (3 × 16) = 54 + 48 = 102 g/mol.

10. How many moles of O₂ are needed to react with 8 moles of C in C + O₂ → CO₂?

4 mol

8 mol

12 mol

16 mol

In the reaction C + O₂ → CO₂, one mole of carbon (C) reacts with one mole of oxygen (O₂) to produce one mole of carbon dioxide (CO₂). Therefore, to react with 8 moles of carbon, 8 moles of oxygen are required, maintaining a 1:1 ratio between C and O₂.

Explanation

In the reaction C + O₂ → CO₂, one mole of carbon (C) reacts with one mole of oxygen (O₂) to produce one mole of carbon dioxide (CO₂). Therefore, to react with 8 moles of carbon, 8 moles of oxygen are required, maintaining a 1:1 ratio between C and O₂.

11. What is the molar mass of NH₄NO₃ (ammonium nitrate)?

60 g/mol

70 g/mol

80 g/mol

90 g/mol

To calculate the molar mass of NH₄NO₃, add the atomic masses of its constituent elements: nitrogen (N) has an atomic mass of about 14 g/mol, hydrogen (H) about 1 g/mol, and oxygen (O) about 16 g/mol. The formula NH₄NO₃ contains two nitrogen atoms, four hydrogen atoms, and three oxygen atoms, resulting in a total of 80 g/mol.

Explanation

To calculate the molar mass of NH₄NO₃, add the atomic masses of its constituent elements: nitrogen (N) has an atomic mass of about 14 g/mol, hydrogen (H) about 1 g/mol, and oxygen (O) about 16 g/mol. The formula NH₄NO₃ contains two nitrogen atoms, four hydrogen atoms, and three oxygen atoms, resulting in a total of 80 g/mol.

12. If 2 moles of CH₄ (methane) are burned, how many moles of CO₂ are produced?

1 mol

2 mol

3 mol

4 mol

Burning methane (CH₄) in oxygen produces carbon dioxide (CO₂) and water (H₂O) according to the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O. For every mole of CH₄ burned, one mole of CO₂ is produced. Thus, burning 2 moles of CH₄ yields 2 moles of CO₂.

Explanation

Burning methane (CH₄) in oxygen produces carbon dioxide (CO₂) and water (H₂O) according to the reaction: CH₄ + 2O₂ → CO₂ + 2H₂O. For every mole of CH₄ burned, one mole of CO₂ is produced. Thus, burning 2 moles of CH₄ yields 2 moles of CO₂.