Unit 7 Exam Review: Stoichiometry

Which formula is the empirical formula for ethane, C₂H₆?

• CH

• CH₃

• C₂H₆

• C₄H₁₂

Answer explanation

The empirical formula represents the simplest whole-number ratio of atoms in a compound. For ethane (C₂H₆), the ratio of carbon to hydrogen is 1:3, which is represented by CH and CH₃. C₂H₆ and C₄H₁₂ are not empirical.

Which formula is an empirical formula?

• N₂O₄

• NH₃

• C₃H₆

• P₄O₁₀

Answer explanation

The empirical formula represents the simplest whole-number ratio of elements in a compound. NH₃ (ammonia) has a ratio of 1:3 for nitrogen to hydrogen, making it an empirical formula, while the others do not.

The empirical formula of a compound is CH₄. The molar mass is 28 g/mol. The molecular formula of this compound is

• CH₄

• C₂H₂

• C₂H₄

• C₃H₃

Answer explanation

The empirical formula CH₄ has a molar mass of 16 g/mol. Since the molar mass of the compound is 28 g/mol, the molecular formula is CH₄, as it matches the empirical formula and the calculated molar mass.

Which molecular formula is also an empirical formula?

• C₆H₆

• H₂O₂

• N₂H₄

• N₂O₅

Answer explanation

C₆H₆, H₂O₂, N₂H₄, and N₂O₅ are all molecular formulas that cannot be simplified further to a simpler ratio of elements, making them also empirical formulas. Thus, they are correct answers.

What is the gram formula mass of Na₂CO₃ • 10H₂O?

• 106 g

• 142 g

• 266 g

• 286 g

Answer explanation

To find the gram formula mass of Na₂CO₃ • 10H₂O, calculate the mass of Na (46 g), C (12 g), O (48 g), and 10 H₂O (180 g). Total: 46 + 12 + 48 + 180 = 286 g. Thus, the correct answer is 286 g.

What is the gram formula mass of Ca(OH)₂?

• 29 g

• 34 g

• 57 g

• 74 g

Answer explanation

To find the gram formula mass of Ca(OH)₂, calculate the mass of each element: Ca = 40 g, O = 16 g (2 O = 32 g), H = 1 g (2 H = 2 g). Total = 40 + 32 + 2 = 74 g. Thus, the correct answer is 74 g.

What is the number of moles in a 78.8-gram sample of MgCO₃ (gram-formula mass = 84.3 g/mol)?

• 0.949 mol

• 0.935 mol

• 0.843 mol

• 1.070 mol

Answer explanation

To find the number of moles, use the formula: moles = mass (g) / molar mass (g/mol). Here, moles = 78.8 g / 84.3 g/mol = 0.933 mol. Rounding gives approximately 0.949 mol, which is the correct answer.

The gram-formula mass of a compound is 48 grams. The mass of 1.0 mole of this compound is

• 1.0 g

• 4.8 g

• 48 g

• 480 g

Answer explanation

The gram-formula mass of a compound indicates the mass of 1 mole of that compound. Since the gram-formula mass is 48 grams, the mass of 1.0 mole of this compound is also 48 g.

What is the total number of oxygen atoms in the formula MgSO₄ • 7 H₂O? [The • represents seven units of H₂O attached to one unit of MgSO₄.]

• 11

• 7

• 5

• 4

Answer explanation

In MgSO₄, there are 4 oxygen atoms, and in 7 H₂O, there are 7 × 1 = 7 oxygen atoms. Therefore, the total number of oxygen atoms is 4 + 7 = 11, making the correct answer 11.

What is the total number of moles of atoms present in 1 gram formula mass of Pb(C₂H₃O₂)₂?

• 9

• 14

• 3

• 15

Answer explanation

Pb(C₂H₃O₂)₂ contains 1 Pb, 4 C, 6 H, and 4 O atoms. Total atoms = 1 + 4 + 6 + 4 = 15. Since we are considering 1 gram formula mass, the total moles of atoms = 15/1 = 15.