Percent Composition and Chemical Formulas

Section 10.3 Part #1

Percent Composition - Definition Percent Composition – the percent by

mass of each element in a compound Expresses the relative amounts of each

element in a compound There are as many % values as there are

different elements in the compound

Percent Composition - Definition % Composition of an Element (E) =

(grams of E ÷ grams of compound) x 100%

The Percent Composition of all elements in the compound added up together must = 100%

Ex. K2CrO4

Ionic Compound, Molecular Compound, or Acid?

Ionic

Name? Potassium Chromate

Made up of: K = 40.3% Cr = 26.8% O = 32.9%

40.3% + 26.8% + 32.9% = 100.0%

But how do YOU find these percentages?

Percent Composition from MassAn 8.20 g piece of Magnesium combines completely with 5.40 g of Oxygen to form a compound. What is the % Composition of this compound?Known:

Mass of Mg = 8.20 g Mass of O = 5.40 g Total mass of compound = 8.20 g + 5.40 g = 13.60 g

% Mg = mass of Mg/mass of compound x 100%= 8.20g Mg / 13.60 g Compound x 100% = 60.3 % Mg

% O = mass of O / mass of compound x 100%

= 5.40 g O / 13.60 g Compound x 100% = 39.7% O

Check: 60.3 % Mg + 39.7 % O = 100.0%

Percent Composition from Formula To calculate the % Composition of a known

compound, use the chemical formula to calculate the molar mass

This gives the mass of 1 mole of the compound Next calculate the % by mass of 1 mole of the

compound for each element Divide the mass of each element in one mole of

the compound by the molar mass and multiply by 100%

% mass = grams of element in 1 mol compound ÷ molar mass of compound x 100%

Example

Calculate the % Composition of Propane (C3H8) Find the molar mass of C3 H8

3(12.01gC) + 8 (1.01gH)

36.03 g C + 8.08 g H = 44.11 g C3 H8

Mass of C in 1 mol C3 H8 = 36.03 g C

Mass of H in 1 mol C3 H8 = 8.08 g H

% C = Mass of C / g of Compound x 100%

= 36.03 g C / 44.11 g C3H8 x 100% = 81.68% C % H = Mass of H / g of Compound x 100%

= 8.08 g H / 44.11 g C3 H8 x 100% = 18.32% H

Using % as a Conversion Factor You can use % Composition to calculate

the # of grams of an element in a specific amount of a compound.

Multiply the mass of the compound by the conversion factor based on % Composition.

From the Previous Example

Ex. Propane: C3 H8

81.68% C, 18.32%H How many grams of Carbon would you have

in a 100g sample of C3 H8 ? 81.68% C is in C3 H8 is like saying 816.8g C/100g C3

H8

100g C3 H8 (81.68 g C/100 g C3 H8) = 81.68 g C

You can use this relationship as a conversion factor. We will use it in the next example.

Example

Calculate the mass of Carbon in 82.0 g of Propane (C3 H8) Known:

Mass of C3 H8 = 82.0 g % Mass of C in C3 H8 = 81.68% We need to find mass of C in g

82.0 g C3 H8 (81.68 g C / 100g C3 H8) = 67.00 g CCheck:

67.00 g C/82.0 g C3 H8 = 81.68% C