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How to find molecular mass?

Since I fell asleep in class, how do you find the molecular formula when the molecular mass is given?

  • I'll share a problem so you could maybe guide me through the problem that way I can solve the other problems using this one. Find the molecular formula of 40.0% C, 6.71% H, ...show more

  • Answer:

    So you have 60 grams/mol. 40% of that is carbon, so 0.40 * 60 grams/mol = 24 grams per mole. Since carbon weights 12 grams/mol, you have 24/12 = 2 carbons. 6.71% hydrogen * 60 grams/mol = 4.026 grams/mole and hydrogen weights 1g/m so 4/1 = 4 hydrogens. 53.29% oxygen * 60 grams/mol = .5329 * 60 = ~ 31.8 grams per mole, oxygen weighs 16 g/m so you have 32/16 = 2 oxygens. So you have C2H4O2 That should be your molecular formula! Hope this helps! P.S. I love how the guy above me originally gave an incorrect answer then stole everyone elses below his... that's great.

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First we need to find the empirical formula, the smallest whole number ratio between the atoms in the compound. Let's say that we have 100 g of the compound. Then according to the percentages given, there are 40.0 g of C, 6.71 g of H, and 53.29 g of O. Convert those to moles by dividing mass by the atomic weight for each element. C: 40.0 g C x (1 mole C / 12.0 g C) = 3.33 moles C H: 6.71 g H x (1 mole H / 1.01 g H) = 6.64 moles H O: 53.29 g O x (1 mole O / 16.0 g O) = 3.33 moles O Dividing each of those by the smallest (3.33) we get C 1 H 2 O 1 The empirical formula is CH2O. The molar mass of CH2O = C (12.0) + 2 H (2 x 1.01) + O (16.0) = 30.0 g/mole. The actual molecular mass = 60 g/mole. Molecular formula = (molecular mass / empirical mass) x empirical formula = (60 / 30) x CH2O = 2 x CH2O = C2H4O2.

HPV

nC=60*40% / atomic mass of C where n would be the number of atoms of C = 24/12 = 2 nH=60*6.71% / am of H = 4/1 = 4 nO=60*53.29 / am of O = 32/16 = 2 C2 H4 O2

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