(40) GCSE Chemistry Revision "Reacting Masses 2"

Understanding Moles and Reacting Masses

In this video, we will focus on using the concept of moles to compute the masses of reactants and products in chemical reactions. This material is intended for higher-tier students.

Key Concepts

• Moles in Reactions: The number of moles in a chemical reaction indicates the proportions of reactants and products.

• Chemical Equations: These depict the reactants and products, including coefficients that represent moles.

Example Calculation 1: Magnesium Chloride from Hydrochloric Acid

Chemical Reaction

• Reaction: 2 HCl + Mg → MgCl2

  • Coefficients: 2 moles of hydrochloric acid (HCl) yield 1 mole of magnesium chloride (MgCl2).

Given Data

• Mass of hydrochloric acid: 146 g

• Relative formula mass of HCl: 36.5

Steps to Calculate

1. Calculate Moles of HCl:

   • Moles = Mass / Relative Formula Mass

   • Moles of HCl = 146 g / 36.5 = 4 moles

2. Use Reaction Ratio:

   • From the equation, 2 moles of HCl make 1 mole of MgCl2.

   • Therefore, 4 moles of HCl will produce 2 moles of MgCl2.

3. Calculate Mass of MgCl2:

   • Relative formula mass of MgCl2 = 95

   • Mass = Moles x Relative Formula Mass = 2 x 95 = 190 g

Final Answer

• Mass of magnesium chloride produced: 190 g

Example Calculation 2: Sodium Sulfate from Sodium Hydroxide

Chemical Reaction

• Reaction: 2 NaOH → Na2SO4

  • Coefficients: 2 moles of sodium hydroxide (NaOH) yield 1 mole of sodium sulfate (Na2SO4).

Given Data

• Mass of sodium hydroxide: 240 g

• Relative formula mass of NaOH: 40

Steps to Calculate

1. Calculate Moles of NaOH:

   • Moles = 240 g / 40 = 6 moles

2. Use Reaction Ratio:

   • 2 moles of NaOH make 1 mole of Na2SO4, hence 6 moles of NaOH yields 3 moles of Na2SO4.

3. Calculate Mass of Na2SO4:

   • Relative formula mass of Na2SO4 = 142

   • Mass = 3 x 142 = 426 g

Final Answer

• Mass of sodium sulfate produced: 426 g

Example Calculation 3: Hydrogen Peroxide from Oxygen

Chemical Reaction

• Reaction: 2 H2O2 → 1 O2

  • Coefficients: 2 moles of hydrogen peroxide yield 1 mole of oxygen.

Given Data

• Mass of oxygen: 64 g

• Relative formula mass of O2: 32

Steps to Calculate

1. Calculate Moles of O2:

   • Moles = 64 g / 32 = 2 moles

2. Use Reaction Ratio:

   • From the equation, we need double the moles of H2O2 to produce moles of O2. Thus, 2 moles of O2 require 4 moles of H2O2.

3. Calculate Mass of H2O2:

   • Relative formula mass of H2O2 = 34

   • Mass = 4 x 34 = 136 g

Final Answer

• Mass of hydrogen peroxide required: 136 g

Conclusion

• By understanding the relationship between moles in chemical equations and using relative formula masses, students can accurately calculate the masses of substances involved in reactions.