Molar Mass (College Board AP® Chemistry)

How many molecules of C₆H₁₄ does a 42.0 g sample of C₆H₁₄ contain?

 Answer:

•  Molar mass of C₆H₁₄

  • M = (6 × 12.01 g/mol) + (14 × 1.008 g/mol)

  • M = 72.06 g/mol + 14.112 g/mol

  • M = 86.172 g/mol

•  Number of moles of C₆H₁₄ 

  • 

  • 

  • 

•  Number of molecules of C₆H₁₄

  • Number of molecules = N_A × n

  • Number of molecules = 6.022 × 10²³ mol^-1 × 0.4874 mol

  • Number of molecules = 2.94 × 10²³

 Alternate Answer

•  Molar mass of C₆H₁₄

  • M = (6 × 12.01 g/mol) + (14 × 1.008 g/mol)

  • M = 72.06 g/mol + 14.112 g/mol

  • M = 86.172 g/mol

•  Solve for number of molecules of C₆H₁₄ using the factor-label method

  • 

The factor-label method uses conversions between units that are equivalent to convert from one unit to another. The original number is multiplied by a fraction made up of the equivalent units so that the units of the original number cancel with one of the units in the conversion. If this leads to the units for the answer then the problem is complete. If not, another conversion can be added.