calculations

• Calculating the molecular formula of a hydrated ionic salt:

  E.g "Calculate the formula of a hydrated magnesium sulfate, MgSO4•xH2O, given that heating 24.6 g of the hydrated salt leaves 12.0 g of anhydrous MgSO4."

  Step 1: Calculate mass of water lost

  Initial mass - Final mass

  
  

  Step 2: Calculate moles of anhydrous salt remaining (MgSO4)

  m/mr

  
  

  Step 3: Calculate moles of water lost

  m/mr

  
  

  Step 4: Determine the simplest H2O : MgSO4 mole (salt-water) ratio

  H2O : MgSO4 = 0.700 : 0.100 = 7 : 1

  x = 7

  
  

  Step 5: Write the molecular formula

  MgSO4•7H2O.
• Determining the molecular formula from empirical formula:

  E.g "A compound has an empirical formula of CH and a molar mass of 78.0 g mol-1. Determine its molecular formula."

  Step 1: Calculate empirical formula mass

  Empirical formula mass = 12.0 + 1.0 = 13.0 g mol-1

  Step 2: Divide molecular mass by empirical formula mass

  mr/empirical formula mass ​= 78/13 ​=6.0

  Step 3: Determine the number of empirical units per molecule

  The compound contains 6 empirical units per molecule.

  Step 4: Write the molecular formula

  C6H6.
• Calculating empirical formula from combustion data:

  E.g "A hydrocarbon sample, 2.20 g, was completely combusted in O2, producing 3.30 g of CO2 and 1.35 g of H2O.

  Determine the empirical formula of the hydrocarbon."

  Step 1: Calculate moles of CO2 produced

  Step 2: Calculate moles of C in CO2

  1 atom of C

  Step 3: Calculate moles of H2O produced

  Step 4: Calculate moles of H in H2O

  2 atoms of H

  2 x 0.075 = 0.150 mol

  Step 5: Determine the simplest C : H mole ratio

  0.075 : 0.150 = 1 : 2

  Step 6: Write the empirical formula 

  CH2
• Calculating empirical formula from percentage composition:

  E.g "Calculate the empirical formula for a compound with 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass."

  Step 1: Assume 100 g of the compound

  percentages equal to grams so stay the same

  Step 2: Convert mass to moles

  each element/own Ar

  Step 3: Determine the simplest mole ratio

  Divide all mole quantities by the smallest number of moles calculated

  Step 4: Write the empirical formula

  CH2O.