Stoichiometry Questions

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Bring on the tough stuff

1. Copper has two stable isotopes ⁶³₂₉Cu (atomic mass = 62.93 amu; 69.09% natural abundance) and ⁶⁵₂₉Cu (atomic mass = 64.9278 amu; 30.91 % natural abundance). Calculate the average atomic mass of copper.

2. How many atoms of sulfur are in 25.1 g of S?

3. Calculate the molecular mass (in amu) of caffeine (C₈H₁₀N₄O₂).

4. How many hydrogen atoms are present in 43.8 g of urea [(NH₂)₂CO]?

5. Phosphoric acid is H₃PO₄. Calculate the percent composition by mass of H, P, and O in this compound.

6. Balance the following equation with smallest whole number coefficients:

Al + O₂ → Al₂O₃

7. Consider the reaction: C₆H₁₂O₆ + 6 O₂ → 6 CO₂+ 6 H₂O. If 968 g of C₆H₁₂O₆ is consumed, what is the mass of CO₂ produced?

8. Consider the following reaction: 2 Li + 2 H₂O → 2 LiOH + H₂. How many grams of Li are needed to produce 7.79 g of H₂?

9. Consider the following equation: 2 NH₃ + CO₂ → (NH₂)₂CO + H₂O. If 849.2 g of NH₃ is treated with 1223 g of CO₂ how much (NH₂)₂CO is formed?

10. Consider the following reaction: C₆H₁₂O₆+ 6 O₂ → 6 CO₂ + 6 H₂O. How many grams of oxygen are needed to make 550 grams of CO₂?

Possible Answers

1. Copper has two stable isotopes ⁶³₂₉Cu (atomic mass = 62.93 amu; 69.09% natural abundance) and ⁶⁵₂₉Cu (atomic mass = 64.9278 amu; 30.91 % natural abundance). Calculate the average atomic mass of copper.

Each isotope contributes to the average atomic mass based on its relative abundance. Multiplying the mass of an isotope by its fractional abundance will give the contribution to the average atomic mass of that isotope.

(0.6909)(62.93 amu) + (0.3091)(64.9278 amu) = 63.55 amu

2. How many atoms of sulfur are in 25.1 g of S?

We need two conversions: first from grams to moles and then from moles to number of particles (atoms).

The conversion factor is:

Avogadro's number is also key:

1 mole = 6.023 × 10²³ particles (atoms)

To solve we need to:

grams of S → moles of S → number of S atoms

3. Calculate the molecular mass (in amu) of caffeine (C₈H₁₀N₄O₂).

To calculate molecular mass, we need to sum all the atomic masses in the molecule. For each element, we multiply the atomic mass of the element by the number of atoms of that element in the molecule. We can find the atomic masses on the periodic table.

There are eight C atoms, ten H atoms, four N atoms, and two O atoms in caffeine, so the molecular mass is given by:

8(12.01 amu) + 10(1.008 amu) + 4(14.01 amu) + 2(16.00 amu) = 194.20 amu

4. How many hydrogen atoms are present in 43.8 g of urea [(NH₂)₂CO]?

We first need to calculate the molar mass of urea: 60.06 g

grams of urea → moles of urea → moles of H → atoms of H

5. Phosphoric acid is H₃PO₄. Calculate the percent composition by mass of H, P, and O in this compound.

Assume that you have 1 mole of H₃PO₄. The percent by mass of each element is given by the combined molar mass of the atoms of the element in 1 mole of H₃PO₄ divided by the molar mass of H₃PO₄, then multiplied by 100 percent.

The molar mass of H₃PO₄ is 97.99 g. That means the percent by mass of each of the elements is:

6. Balance the following equation with smallest whole number coefficients:

Al + O₂ → Al₂O₃

Al +3 O₂ → 2 Al₂O₃

4 Al +3 O₂ → 2 Al₂O₃