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A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular mass of 318 g/mol. What is the molecular formula for this compound if it has a molecular mass of 318 g/mol? Select the correct response​

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JichuSooya03viz

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Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages).

  • (.7546) (100 g) = 75.46 g C
  • (.0443) (100 g) = 4.43 g H

  • (.2010) (100 g) = 20.10 g O

Convert the mass of each element to moles.

  • (75.46 g C) (1 mol/ 12.00 g C) = 6.289 mol C
  • (4.43 g H) (1 mol/ 1.008 g H) = 4.39 mol H

  • (20.10 g O) (1 mol/ 16.00 g O) = 1.256 mol O

Find the ratio of the moles of each element.

  • (1.256 mol O)/ (1.256) = 1 mol O
  • (6.289 mol C)/ (1.256) = 5.007 mol C

  • (4.39 mol H)/ (1.256) = 3.50 mol H

Use the mole ratio to write the empirical fomula.

Multiplying the mole ratios by two to get whole number, the empirical formula becomes:

  • C10H7O2

Find the mass of the empirical unit.

  • 10(12.00) + 7(1.008) + 2(16.00) = 159.06 g/mol

Figure out how many empirical units are in a molecular unit.

  • (318.31 g/mol) / (159.06 g/mol) = 2.001 empirical units per molecular unit

Write the molecular formula.

Since there are two empirical units in a molecular unit, the molecular formula is:

  • C20H14O4

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