Overview
This lesson covers how to use the formula ( n = \frac{m}{M} ) to calculate moles, mass, and molar mass for chemical compounds, using worked examples.
The Mole Formula
- ( n = \frac{m}{M} ) where ( n ) is moles, ( m ) is given mass (grams), ( M ) is molar mass (g/mol).
- Small ( m ) = mass given in the problem; capital ( M ) = mass per mole from periodic table.
- Molar mass is found by adding atomic masses of all atoms in the compound, expressed in grams per mole (g/mol).
Worked Examples
- 88 g CO(_2): Molar mass = 12 (C) + 2×16 (O) = 44 g/mol → ( n = \frac{88}{44} = 2 ) mol.
- 54 g H(_2)O: Molar mass = 2×1 (H) + 16 (O) = 18 g/mol → ( n = \frac{54}{18} = 3 ) mol.
- 49 g H(_2)SO(_4): Molar mass = 2×1 (H) + 32 (S) + 4×16 (O) = 98 g/mol → ( n = \frac{49}{98} = 0.5 ) mol.
- To find mass when moles are given: ( m = n \times M ).
Using the Formula Triangle
- Arrange ( m ) at the top, with ( n ) and ( M ) at the bottom corners.
- To find ( m ): ( m = n \times M ).
- To find ( n ): ( n = \frac{m}{M} ).
Additional Example Calculations
- 3 mol CO(_2): Molar mass = 44 g/mol → ( m = 3 \times 44 = 132 ) g.
- 9 mol NaOH: Molar mass = 23 (Na) + 16 (O) + 1 (H) = 40 g/mol → ( m = 9 \times 40 = 360 ) g.
- 50 g CaCO(_3): Molar mass = 40 (Ca) + 12 (C) + 3×16 (O) = 100 g/mol → ( n = \frac{50}{100} = 0.5 ) mol.
Key Terms & Definitions
- Mole (n) — Quantity that measures the amount of substance; unit is "mol".
- Mass (m) — Actual mass of the substance given, in grams (g).
- Molar mass (M) — Mass of one mole of a substance in grams (g/mol), found using the periodic table.
Action Items / Next Steps
- Practice using the ( n = \frac{m}{M} ) formula with different compounds and scenarios.
- Review the method for calculating molar mass using the periodic table.