Chemistry Quantitative Chemistry

What does the law of conservation of mass state?

• No atoms are lost or made during a chemical reaction.

• The mass of the products equals the mass of the reactants.

Why might the mass appear to change during a reaction?

• A gas might be produced and escape into the air.

• A gas from the air might be reacting (e.g., oxygen reacting with a metal).

What is relative formula mass (Mr), and how is it calculated?

• The sum of the relative atomic masses (Ar) of all atoms in a compound.

• For example, Mr of H₂O = (2 × 1) + (1 × 16) = 18.

What is a mole in chemistry?

• A unit measuring chemical amounts.

• One mole contains 6.02 × 10²³ particles (Avogadro’s number).

How do you calculate the mass of a substance from moles?

Mass (g) = Moles × Molar Mass (g/mol).

How can balanced equations be used to determine reacting masses?

• The coefficients in a balanced equation show the mole ratio of reactants and products.

• Example: 2H₂ + O₂ → 2H₂O (2 moles of H₂ react with 1 mole of O₂ to form 2 moles of H₂O).

What is a limiting reactant?

• The reactant that is completely used up first in a reaction.

• Determines the amount of product formed.

What is percentage yield and how is it calculated?

• (Actual yield / Theoretical yield) × 100.

• Measures how much product is obtained compared to the maximum possible amount.

Why is actual yield often less than theoretical yield?

• Reaction may not go to completion.

• Side reactions may occur.

• Losses during purification (e.g., filtration, transfer, evaporation).

What is atom economy, and why is it important?

• Atom economy (%) = (Mr of desired product / Total Mr of all reactants) × 100.

• High atom economy = Less waste, more sustainable, cost-effective.

What is concentration, and how is it calculated in g/dm³?

Concentration = Mass of solute (g) / Volume of solution (dm³).

How is concentration calculated in mol/dm³?

Concentration (mol/dm³) = Moles of solute / Volume of solution (dm³).

How do you convert concentration between g/dm³ and mol/dm³?

Concentration (mol/dm³) = Concentration (g/dm³) / Molar Mass (g/mol).

What is the equation for calculating moles in a solution?

Moles = Concentration (mol/dm³) × Volume (dm³).

How do you calculate the volume of a gas at room temperature from moles?

Volume (dm³) = Moles × 24.

How can you determine the volume of gas produced in a reaction?

• Use the balanced equation to find the mole ratio.

• Calculate moles of reactant.

• Use Volume = Moles × 24 dm³ to determine the gas volume.

How to convert from cm³ to dm³?

Divide by 1000.

How to calculate uncertainty?

Range / 2.

How to find number of particles in a substance?

Moles * 6.02*10²³.

How to find mean rate of reaction?

Quantity of product formed or reactants used / time taken.