calculations

    Cards (4)

    • 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.
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