Naphthalene, commonly found in mothballs , is commonly found in 93.7% carbon and 6.3% hydrogen. The molar mass of naphthalene is 128g/mol. Determine the empirical and molecular formulas for naphthalene
assume 1000 grams of the stuff.
937 grams of C,or 937/12 moles C=78 moles
63 grms of H, or 63moles of H.
normalizing, 78/63 moles of C for every mole of H
C: 1.24 H: 1 is the empirical formula, but we want to put it in whole numbers, so finding a multiple of 1.24 giving a whole number, 4 is a starting pint C5H4 is a starting point, it is the empirical formula.
but we have more information: mole mass of 128g/mole
mole mass of empirical C5H4 is (5*12+4)=64, so we know the molar mass is twice the empirical formula:
molecular formula: C10H8
To determine the empirical formula of naphthalene, we need to find the simplest whole number ratio of the elements present (carbon and hydrogen) in the compound.
Step 1: Determine the number of moles of each element.
Given the percentages, we can assume we have a 100g sample of naphthalene.
Molar mass of carbon (C) = 12.01 g/mol
Molar mass of hydrogen (H) = 1.008 g/mol
Number of moles of carbon = (93.7g C) / (12.01 g/mol C) ≈ 7.80 mol C
Number of moles of hydrogen = (6.3g H) / (1.008 g/mol H) ≈ 6.25 mol H
Step 2: Divide each mole value by the smallest mole value to get the simplest ratio.
Dividing by 6.25 (the smallest value), we get:
C ≈ 7.80 mol C / 6.25 ≈ 1.25
H ≈ 6.25 mol H / 6.25 ≈ 1
Step 3: Apply whole number ratios to the elements, giving us the empirical formula.
The empirical formula is therefore C1.25H1, which can be simplified to CH.
To determine the molecular formula, we need additional information—a comparison between the empirical formula mass and the molar mass of naphthalene.
Molar mass of naphthalene = 128 g/mol
Step 4: Calculate the empirical formula mass.
Empirical formula mass = (12.01 g/mol × 1) + (1.008 g/mol × 1) = 13.02 g/mol
Step 5: Divide the molar mass of naphthalene by the empirical formula mass.
Molar mass of naphthalene / Empirical formula mass = 128 g/mol / 13.02 g/mol ≈ 9.81
Step 6: Multiply the subscripts in the empirical formula by the value calculated in step 5.
(CH)9.81 = C9.81H9.81
Therefore, the molecular formula of naphthalene is approximately C9.81H9.81.