Stoichiometry example problem 1 | Physical Processes | MCAT | Khan Academy

Summary

The video explains the stoichiometry of the reaction between solid phosphorus and chlorine gas to form phosphorus trichloride. It starts by balancing the chemical equation, ensuring equal atoms on both sides. The process involves converting given grams of phosphorus to moles, determining the required moles of chlorine gas, and calculating the grams needed. Finally, it addresses how to find the amount of phosphorus trichloride produced, emphasizing mass conservation. The explanation includes detailed steps and unit cancellations, making the complex chemistry problem accessible.

Takeaways

• 🧪 The reaction between solid phosphorus and chlorine gas produces phosphorus trichloride.
• 🔄 Stoichiometry is essential for determining the quantities of reactants and products in a chemical reaction.
• ⚖️ The chemical equation must be balanced to ensure the correct stoichiometric ratios.
• 🔍 The atomic weight of phosphorus (P) is approximately 31, and for P4, it's 124 grams per mole.
• 📊 To find the moles of a substance, divide the mass by the molar mass.
• 📚 Moles are a measure of the amount of substance, with one mole containing Avogadro's number of entities.
• 🔄 For every mole of P4, six moles of Cl2 are required according to the balanced chemical equation.
• 📈 The molar mass of Cl2 is approximately 70.906 grams per mole, used to calculate the mass of chlorine needed.
• 📝 The calculation of moles of reactants and products is crucial for understanding the chemical reaction.
• 🧐 Conservation of mass is applied to find the mass of the product, phosphorus trichloride, by adding the masses of the reactants.
• 📉 The total mass of reactants equals the total mass of the products, indicating no mass is lost or gained in the reaction.

Q & A

What is the balanced chemical equation for the reaction between phosphorus and chlorine gas?

The balanced chemical equation is 4P + 6Cl2 → 4PCl3, representing the reaction of solid phosphorus with chlorine gas to produce phosphorus trichloride.

What is the molar mass of phosphorus (P) in grams per mole?

The molar mass of phosphorus is approximately 31 grams per mole.

How many grams of a P4 molecule would be equivalent to one mole?

One mole of P4, which contains four phosphorus atoms, would be 124 grams (4 times the molar mass of a single phosphorus atom).

How many moles of P4 are present in 1.45 grams of solid molecular phosphorus?

There are 0.0117 moles of P4 in 1.45 grams of solid molecular phosphorus, calculated by dividing the mass by the molar mass of P4.

What is the stoichiometric ratio of chlorine gas to phosphorus in the balanced equation?

The stoichiometric ratio is 6 moles of chlorine gas to 1 mole of phosphorus.

How many moles of chlorine gas are required to react with 0.0117 moles of P4?

0.07 moles of chlorine gas are required, based on the stoichiometric ratio from the balanced equation.

What is the molar mass of chlorine gas (Cl2) in grams per mole?

The molar mass of chlorine gas is approximately 70.906 grams per mole.

How many grams of chlorine gas are needed to react with 1.45 grams of phosphorus?

4.96 grams of chlorine gas are required, calculated by multiplying the moles of chlorine gas needed (0.07 moles) by its molar mass.

What is the law of conservation of mass, and how does it apply to this chemical reaction?

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. In this reaction, the total mass of reactants (phosphorus and chlorine gas) equals the mass of the product (phosphorus trichloride).

How can you calculate the mass of phosphorus trichloride produced in the reaction?

You can either use the stoichiometry of the reaction to find the moles of PCl3 produced and then convert to mass, or apply the law of conservation of mass, which states the mass of the product will be the sum of the masses of the reactants.

What is the significance of balancing a chemical equation before performing stoichiometry calculations?

Balancing a chemical equation ensures that the number of atoms of each element is the same on both sides of the equation, which is necessary for accurate stoichiometry calculations and reflects the actual chemical reaction taking place.