Stoichiometry: Videos & Practice Problems

Balancing chemical equations is crucial for understanding stoichiometry, which explores the numerical relationships between reactants and products. For instance, in the reaction of 2 H₂ + O₂ → 2 H₂O, knowing the mass of H₂ allows us to calculate the mass of H₂O produced. This involves converting grams to moles, using mole ratios from the balanced equation, and then converting back to grams. Mastering this process is essential for accurate stoichiometric calculations in chemistry.

Stoichiometry

Balancing chemical equations is a crucial first step in understanding stoichiometry, which focuses on the quantitative relationships between reactants and products in a chemical reaction. Stoichiometry enables us to calculate the amounts of products formed from given reactants and vice versa. For instance, consider the balanced chemical equation:

2 H₂ + O₂ → 2 H₂O

In this example, 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water. If we know that 12.3 grams of H₂ are used, we can determine the mass of water (H₂O) produced through stoichiometric calculations.

To perform these calculations, we first need to ensure that the chemical equation is balanced. This balance allows us to use the mole ratios derived from the coefficients in the equation to convert between the amounts of different substances. The next step involves applying the appropriate procedures for stoichiometric calculations, which will be explored further in subsequent discussions.

Stoichiometry

To determine how many grams of water (H₂O) can be formed from 12.3 grams of hydrogen gas (H₂), we utilize the principles of stoichiometry. Stoichiometry allows us to relate the quantities of reactants and products in a chemical reaction through a balanced equation. The first step is to convert the mass of hydrogen gas into moles, as stoichiometric calculations are typically performed in terms of moles.

The conversion from grams to moles is achieved using the molar mass of hydrogen, which is approximately 2.02 g/mol. Thus, the number of moles of H₂ can be calculated using the formula:

\[\text{Moles of } H_2 = \frac{\text{Mass of } H_2}{\text{Molar Mass of } H_2} = \frac{12.3 \text{ g}}{2.02 \text{ g/mol}} \approx 6.09 \text{ moles}\]

Next, we need to refer to the balanced chemical equation for the reaction between hydrogen and oxygen to form water:

\[2H_2 + O_2 \rightarrow 2H_2O\]

From the balanced equation, we see that 2 moles of H₂ produce 2 moles of H₂O. This indicates a 1:1 mole ratio between H₂ and H₂O. Therefore, the moles of water produced can be calculated as follows:

\[\text{Moles of } H_2O = \text{Moles of } H_2 \times \frac{2 \text{ moles } H_2O}{2 \text{ moles } H_2} = 6.09 \text{ moles } H_2O\]

Finally, to find the mass of water produced, we convert moles of water back to grams using the molar mass of water, which is approximately 18.02 g/mol:

\[\text{Mass of } H_2O = \text{Moles of } H_2O \times \text{Molar Mass of } H_2O = 6.09 \text{ moles} \times 18.02 \text{ g/mol} \approx 109.1 \text{ g}\]

In summary, starting with 12.3 grams of hydrogen gas, we can produce approximately 109.1 grams of water through the stoichiometric process, which involves converting grams to moles, using the mole ratio from the balanced equation, and converting back to grams.

Stoichiometry Example 1

In stoichiometry, we often need to convert between the amounts of different substances in a chemical reaction. This process involves using a balanced chemical equation to relate the quantities of reactants and products. For example, if we want to determine how many grams of water (H₂O) are produced from a given mass of hydrogen gas (H₂), we can follow a systematic approach.

First, we start with the mass of the given substance, which in this case is 12.3 grams of H₂. The first step is to convert this mass into moles. To do this, we use the molar mass of H₂, which is calculated as follows:

For H₂: 2 × 1.008 g/mol = 2.016 g/mol

Using this molar mass, we convert grams to moles:

Number of moles of H₂ = \(\frac{12.3 \text{ g H}_2}{2.016 \text{ g/mol}} \approx 6.1012 \text{ moles H}_2\)

Next, we need to relate the moles of H₂ to the moles of H₂O produced. According to the balanced equation for the reaction, 2 moles of H₂ produce 2 moles of H₂O. This means the mole ratio is 1:1. Therefore, the moles of H₂O produced will also be 6.1012 moles:

Number of moles of H₂O = 6.1012 moles

Finally, we convert the moles of H₂O into grams. The molar mass of H₂O is calculated as follows:

For H₂O: (2 × 1.008 g/mol) + (16.00 g/mol) = 18.016 g/mol

Now, we can find the mass of H₂O produced:

Mass of H₂O = \(6.1012 \text{ moles} \times 18.016 \text{ g/mol} \approx 109.9 \text{ g H}_2\text{O}\)

Since the original measurement (12.3 g) has three significant figures, we should express our final answer in the same way. Thus, the mass of water produced is approximately:

1.10 × 10² g H₂O

This example illustrates the importance of stoichiometry in converting between different substances in a chemical reaction, emphasizing the need to use molar masses and balanced equations effectively.

The oxidation of chromium solid is represented by the following equation:

How many moles of chromium (III) oxide are produced when 34.69 g Cr reacts with excess oxygen gas?

• A. 0.334 mol
• B. 1.33 mol
• C. 0.749 mol
• D. 3.00 mol
• E. 0.667 mol

Answer: (A)

The reaction of potassium chlorate and sucrose is given below:

If 2.33 x 10^-7 formula units of potassium chlorate are reacted, how many grams of carbon dioxide will be produced?

• A. 3.94x10^-16 g CO₂
• B. 8.23x10^-23 g CO₂
• C. 2.72x10^-21 g CO₂
• D. 7.81x10^-23 g CO₂
• E. 2.55x10^-29 g CO₂

Answer: (E)

If the density of ethanol, CH₃CH₂OH, is 0.789 g/mL, how many milliliters of ethanol are needed to produce 4.8 g of H₂O in the following reaction?

• A. 5.2 mL
• B. 47 mL
• C. 3.2 mL
• D. 29 mL
• E. 74 mL

Answer: (A)

Here’s what students ask on this topic:

What is stoichiometry and why is it important in chemistry?

Stoichiometry is the study of the numerical relationships between reactants and products in a balanced chemical equation. It allows chemists to predict the amounts of substances consumed and produced in a reaction. For example, in the reaction 2 H₂ + O₂ → 2 H₂O, stoichiometry helps determine how much water (H₂O) can be produced from a given amount of hydrogen (H₂). This is crucial for tasks such as scaling up reactions for industrial production, ensuring reactants are used efficiently, and minimizing waste.

How do you balance a chemical equation?

To balance a chemical equation, follow these steps: 1) Write the unbalanced equation. 2) Count the number of atoms of each element on both sides of the equation. 3) Adjust the coefficients (the numbers in front of the compounds) to balance the atoms for each element. 4) Repeat the process until all elements are balanced. For example, to balance H₂ + O₂ → H₂O, you would adjust it to 2 H₂ + O₂ → 2 H₂O.

How do you convert grams to moles in stoichiometry?

To convert grams to moles, use the formula: moles = grams / molar mass. The molar mass is the mass of one mole of a substance, typically found on the periodic table. For example, to convert 12.3 grams of H₂ to moles, you would use the molar mass of H₂ (2.02 g/mol): moles = 12.3 g / 2.02 g/mol ≈ 6.09 moles.

What is a mole-to-mole comparison in stoichiometry?

A mole-to-mole comparison involves using the coefficients from a balanced chemical equation to relate the amounts of reactants and products. For example, in the equation 2 H₂ + O₂ → 2 H₂O, the coefficients indicate that 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of H₂O. This ratio helps determine how much of one substance is needed or produced when given a certain amount of another substance.

How do you use a stoichiometric chart to solve problems?

A stoichiometric chart helps visualize the steps needed to convert given information to the desired unknown. Start with the given quantity (e.g., grams of a reactant), convert to moles, use the mole ratio from the balanced equation to find moles of the unknown, and then convert back to the desired unit (e.g., grams of a product). For example, to find the grams of H₂O produced from 12.3 grams of H₂, convert grams of H₂ to moles, use the mole ratio to find moles of H₂O, and then convert to grams of H₂O.