Stoichiometry

Calculations

Weighted Average Atomic Mass

Weighted Average Atomic Mass

• Multiply amu by percentage

• Add values together

• Answer is the average amu

Molar mass

the mass of 1 mole of ___ in grams

Avogadro’s Number and Mole

1 mole = N_A = 6.022 × 10²³

Unit Conversions

• 6.022 × 10²³ atoms / 1 mole

• grams (avg atomic mass) / mole

Natural Abundance

• Average Atomic Mass = (x (#amu₁) ) + ( [1 - x] (#amu₂) )

  • Answer is relative abundance for element 1

• x + (1 - x) = 1

  • Answer is relative abundance for element 2

Percent Composition

• ( n x molar mass of element ) / ( molar mass of compound ) x 100%

  • n is the number of moles of the element in 1 mole of the compound

Calculating the empirical formula from the mass percent composition

1. Assume 100 grams of a compound (percentages then become grams)

2. Calculate the moles of each element

3. Divide by the smallest number to obtain the subscripts in the empirical formula

Converting empirical formulas to molecular formulas

• molecular formula molar mass / empirical formula molar mass

  • Answer is the multiplier of each subscript in empirical formula