Lecture: Converting Between Grams and Moles

Overview

• The lecture explains how to convert between grams and moles using aluminum and sodium chloride as examples.
• Focuses on understanding molar mass and its role in conversions.

Key Concepts

Molar Mass

• Definition: Mass of one mole of a substance in grams.
• Aluminum (Al): Molar mass = 26.98 g/mol.
• Sodium Chloride (NaCl): Molar mass calculated by adding molar masses of sodium and chlorine.
  • Sodium (Na) = 22.99 g/mol
  • Chlorine (Cl) = 35.45 g/mol
  • NaCl = 58.44 g/mol

Converting Moles to Grams

1. Identify the number of moles.
2. Multiply the number of moles by the molar mass of the substance.
   • Example: 4.30 moles of aluminum.
   • Calculation: 4.30 moles x 26.98 g/mol = 116 grams (rounded to 3 significant figures).

Converting Grams to Moles

1. Identify the mass in grams.
2. Divide the mass by the molar mass to find the number of moles.
   • Example: 127.5 grams of NaCl.
   • Calculation: 127.5 g / 58.44 g/mol = 2.182 moles (rounded to 4 significant figures).

Conversion Factors

• Purpose: Facilitate conversion by setting up unit cancellations.
• Example for Aluminum:
  • Conversion factor: 26.98 g/mol or 1 mole/26.98 g.
  • Choose based on what needs to be canceled.

Conversion Process

• From Moles to Grams: Multiply moles by molar mass.
• From Grams to Moles: Divide grams by molar mass.
• Both methods (thinking through it vs. conversion factors) result in the same mathematical operation.

Example Problems

• Aluminum: Convert 4.30 moles to grams.
• Sodium Chloride: Convert 127.5 grams to moles.

Significance of Significant Figures

• Always round the final answer to the correct number of significant figures based on given data.

Conclusion

• Understanding and applying the concept of molar mass is crucial for these conversions.
• Both intuitive thinking and structured conversion factors are valid methods.