Stoichiometry: Videos & Practice Problems

Stoichiometry is the study of the quantitative relationships in a balanced chemical equation, allowing us to predict the amount of product formed from given reactants or vice versa. To perform stoichiometric calculations, one must first ensure the chemical equation is balanced. The process involves converting the given quantity of a substance, which could be in grams, ions, atoms, formula units, or molecules, into moles (the 'Given Information'). Using the stoichiometric chart, we move from the given information to the 'Unknown Information' through a mole-to-mole comparison, guided by the coefficients in the balanced equation. This step is crucial and is often referred to as the "jump." Depending on the question, the calculation may stop at moles of the unknown substance or continue to convert back to grams, ions, atoms, formula units, or molecules of the unknown. This systematic approach is essential for accurately determining the mass or number of particles of a product or reactant in a chemical reaction.

Stoichiometry Concept

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 (g) + O₂ (g) → 2 H₂O (g)

In this example, 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water vapor. If we know that 12.3 grams of H₂ are involved in the reaction, we can use stoichiometry to determine the mass of water (H₂O) produced.

To perform stoichiometric calculations, one must follow a systematic approach that involves using the coefficients from the balanced equation to relate the quantities of different substances. This process will be explored further in subsequent discussions, where specific procedures for conducting stoichiometric calculations will be outlined.

Stoichiometric Chart Concept

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 in this process is to convert the mass of the given substance (H₂) into moles.

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

\[\text{Moles of } H_2 = \frac{\text{grams of } H_2}{\text{molar mass of } H_2} = \frac{12.3 \text{ g}}{2.02 \text{ g/mol}} \approx 6.09 \text{ moles}\]

Next, we need to use the balanced chemical equation for the reaction between hydrogen and oxygen to form water. The balanced equation is:

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

This equation indicates that 2 moles of hydrogen gas produce 2 moles of water. From this, we can establish a mole-to-mole ratio. For every 2 moles of H₂, 2 moles of H₂O are produced, which simplifies to a 1:1 ratio. Therefore, the moles of water produced will also be approximately 6.09 moles.

To find the mass of water produced, we convert moles of H₂O back to grams using the molar mass of water, which is approximately 18.02 g/mol:

\[\text{Grams of } H_2O = \text{moles of } H_2O \times \text{molar mass of } H_2O = 6.09 \text{ moles} \times 18.02 \text{ g/mol} \approx 109.1 \text{ g}\]

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, applying the mole ratio from the balanced equation, and converting back to grams.

Stoichiometry Example

To determine how many grams of nitrogen monoxide (NO) are produced from the reaction of 15 grams of nitrogen (N₂), we can follow a systematic approach using stoichiometry. The first step involves identifying the balanced chemical equation for the reaction, which is essential for understanding the mole ratios between reactants and products.

Next, we convert the given mass of nitrogen (N₂) into moles. The molar mass of nitrogen gas (N₂) is calculated as follows: since nitrogen has an atomic mass of approximately 14.01 grams/mol, the molar mass of N₂ is:

M(N₂) = 2 × 14.01 g/mol = 28.02 g/mol

Using this molar mass, we can convert grams of N₂ to moles:

Moles of N₂ = 15 g N₂ × (1 mol N₂ / 28.02 g N₂) ≈ 0.535 moles N₂

Now, we perform a mole-to-mole conversion using the coefficients from the balanced equation. For every 1 mole of N₂, 2 moles of NO are produced. Thus, we can calculate the moles of NO produced:

Moles of NO = 0.535 moles N₂ × (2 moles NO / 1 mole N₂) = 1.07 moles NO

Finally, we convert the moles of NO back into grams. The molar mass of nitrogen monoxide (NO) is the sum of the atomic masses of nitrogen and oxygen (approximately 16.00 g/mol for oxygen):

M(NO) = 14.01 g/mol + 16.00 g/mol = 30.01 g/mol

Now, we can find the mass of NO produced:

Mass of NO = 1.07 moles NO × 30.01 g/mol ≈ 32.1 g NO

In conclusion, when 15 grams of nitrogen reacts, approximately 32.1 grams of nitrogen monoxide are produced, rounded to three significant figures to match the precision of the given data.

The combination reaction for the formation of hydrazine is shown below:
4 NH₃ (g) + N₂ (g) → 3 N₂H₄ (l)
Determine the number of moles of hydrazine formed when 25.7 g ammonia reacts with excess nitrogen gas.

• A. 1.13 mol
• B. 0.567 mol
• C. 0.881 mol
• D. 2.81 mol

Answer: (A)

The reaction between solid phosphorus and fluorine is given below:
P₄ (s) + 12 F₂ (g) → 4 PF₆ (aq)
If 6.92 x 10^-5 molecules of solid phosphorus react to completion, how many grams of phosphorus hexafluoride would be produced?

• A. 5.03 x 10^-26 g
• B. 7.14 x 10^-26 g
• C. 8.19 x 10^-26 g
• D. 6.65 x 10^-26 g

Answer: (D)

Determine the density of 50.0 mL iron (II) bromide when the following chemical reaction produced 112.7 g iron (II) phosphate.
2 K₃PO₄ (aq) + 3 FeBr₂ (aq) → Fe₃(PO₄)₂ (s) + 6 KBr (aq)

• A. 2.31 g/mL
• B. 6.59 g/mL
• C. 4.08 g/mL
• D. 3.95 g/mL

Answer: (C)

Here’s what students ask on this topic:

What is stoichiometry?

Stoichiometry is a section of chemistry that involves the quantitative relationships between the reactants and products in a chemical reaction. It is based on the conservation of mass and the concept that elements are neither created nor destroyed in chemical reactions. Stoichiometry allows chemists to predict the amounts of substances consumed and produced in a given reaction.

To perform stoichiometric calculations, you'll need to use the balanced chemical equation for the reaction. A balanced equation shows the correct proportions of reactants and products, adhering to the law of conservation of mass. From this, you can determine the mole ratio between the substances, which then enables you to calculate how much of one substance is needed to react with a given amount of another, or how much product will be formed from given reactants. This is essential for tasks such as scaling up reactions for industrial purposes, determining yields in laboratory experiments, and analyzing the composition of samples in analytical chemistry.

How to do stoichiometry?

Stoichiometry is a method in chemistry that relates the quantities of substances in a chemical reaction. To perform stoichiometry, follow these steps:

1. Write a Balanced Chemical Equation: Ensure the number of atoms for each element is the same on both sides of the equation.
2. Convert Units to Moles: Use the molar mass of the substances to convert grams to moles if necessary.
3. Use Mole Ratios: From the balanced equation, determine the mole ratio between the reactants and products.
4. Calculate Moles of Unknown: Using the mole ratio and the moles of the known substance, calculate the moles of the unknown substance.
5. Convert Moles to Desired Units: If the problem asks for grams or liters (for gases at STP), convert the moles of the unknown substance to these units using molar mass or molar volume (22.4 L/mol for gases at STP).

Remember, the key to stoichiometry is the balanced chemical equation and the mole ratio it provides, which serves as the conversion factor between reactants and products. Practice with various problems to become proficient in this method.

How is stoichiometry used to calculate the amount of product produced?

Stoichiometry is a branch of chemistry that deals with the quantitative relationships between the reactants and products in a chemical reaction. To calculate the amount of product produced using stoichiometry, you follow these steps:

1. Write down the balanced chemical equation for the reaction. This shows the ratio in which reactants combine and the products formed.
2. Determine the molar mass of the reactants and products using the periodic table.
3. Convert the mass of the reactants used to moles, using their molar mass.
4. Use the mole ratio from the balanced equation to calculate the number of moles of the product that should form.
5. Convert the moles of the product back to mass, if required, using the molar mass of the product.

This process allows you to predict the amount of product that will be produced from a given amount of reactants, assuming the reaction goes to completion and all conditions are ideal. It's a fundamental concept for understanding chemical reactions and is widely used in chemistry to ensure reactions are carried out with the correct proportions of reactants.

How to do mass-to-mass stoichiometry?

Mass-to-mass stoichiometry involves converting the mass of one substance in a chemical reaction to the mass of another substance using the mole ratio from the balanced chemical equation. Here's how to do it:

1. Write the balanced chemical equation for the reaction you are studying. This will provide you with the mole ratio of the reactants and products.
2. Convert the mass of the given substance to moles using its molar mass. The molar mass is the mass of one mole of a substance and can be found on the periodic table (in grams per mole).
3. Use the mole ratio to convert moles of the given substance to moles of the desired substance. The mole ratio comes from the coefficients of the substances in the balanced equation.
4. Convert the moles of the desired substance to mass using its molar mass. Multiply the number of moles by the molar mass of the substance to find the mass.

In summary, the process is: mass of substance A → moles of substance A → moles of substance B → mass of substance B. Always ensure that your units cancel out appropriately during each step, and check your work with dimensional analysis.