Sunday, November 15, 2009

Percentage Mass of Elements in Compounds & Finding Mass of an Element in a Sample.

(November 10, 2009)
Today we learned how to calculate the percentage of mass of elements in compounds. Recall that molar mass is grams over moles: g/mol and molar volume is liters over moles: l/mol.

To find the percentage mass, first find the total molar mass of the compound. Then, for each element in the compound, find its individual molar mass. Divide the element mass with the compound mass:

% of Element = (Element Mass / Compound Mass) x 100

- Example - Compound: Fe3N2
  • In iron (II) nitride, it's molar mass is 195.4 g. *Note that iron has a mass of 55.8g and nitrogen has a mass of 14.0g: 3(55.8) + 2(14.0) = 195.4 g
  • Next, the individual mass of iron is 167.4 g (3 x 55.8 g), nitride is 28.0 g (2 x 14.0 g)
  • Now find the percentage mass by taking element's mass and divide it by the compound's mass.
  • % of Fe = (167.4 g / 195.4 g) x 100 = 85.7%
  • % of N = (28.0 g / 195.4 g) x 100 = 14.3 %
  • To ensure the percentages are accurate, add them together. If their sum adds up to 100, it is accurate. If their sum adds up to a couple of decimals off, that is because of the rounding. Try to adjust the decimals so that your sum will equal 100.

To find the mass of an element in a given sample, first find the total molar mass of the compound. Then, for each element in the compound, find its individual molar mass. Divide the element mass with the compound mass. Lastly, multiply the quotient with the mass of the given sample:

- Example - Compound: 62.0 g sample of ZnO
  • In zinc oxide, the molar mass is 81.4 g. *Note that zinc has a mass of 65.4 g and oxygen has a mass of 16.0 g: 1(65.4 g) + 1(16.0 g) = 81.4 g
  • Next, the individual mass of zinc is 653.4 g (1 x 65.4 g) and oxygen is 16.0 g (1 x 16.0)
  • Now divide the individual masses with the compound mass
  • Zn = (65.4 g / 81.4 g) = .8034
  • O = (16.0 g / 81.4 g) = .1966
  • Then multiply the quotients with the mass of the given sample (62.0 g).
  • Zn = (.8034)(62.0 g) = 49.8 g
  • O = (.1966)(62.0 g) = 12.2 g
  • Therefore, in a 62.0 g sample of ZnO, there are 49.8 g of zinc and 12.2 g of oxygen.
  • To ensure masses are correct, add them together so that they equal the mass of the given sample. If they do not equal the mass of the sample, that may because of the rounding. Adjust the decimals so that the sum will equal the given mass of the sample.

Here is a tutorial clip I found:


That is all. Mr. Doktor is the best!

-- Jael Lumba

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