3.2 Determining Empirical and Molecular Formulas

The elemental makeup of a compound

defines its chemical identity. Chemical formulas are the best way of representing this elemental makeup.

When a compounds formula is unknown

measuring the mass of each of its constituent elements is the first step in the profess of determining the formula experimentally.

The Percent Mass of all elements in a substance add up to…

100%

Percent Composition

the percentage by mass of each element in the compound. EX: A gaseous compound composed solely of Carbon and Hydrogen. The percent composition of this compound is %H = mass H/Mass Compound X 100%. (see image attached).

Percent Composition is useful for evaluating…

the relative abundance of a given element in different compounds of known formula.

An Object is 200g of brass. How many grams of Copper and Zinc does it contain? (Copper’s percent composition is 85% in brass)

0.85 × 200g = 170g of Copper (turned its percentage into a decimal to multiply) Mass of Zinc = Total Mass - Mass of Copper, 200g - 170g = 30g of Zinc, which makes up 15% of Brass since the other 85% is all Copper.

Percent Mass =

mass due to element/total mass X 100%

An object has a mass of 281g and it contains 43g of Al. What is the percent mass of Al in the object?

(Remember to use Percent Mass Formula) 43g of Al/281g of compound = 15% of Al. (Your calculator will show 0.149466 but you round up to 2 significant figures, giving you 15%).

How do you find % mass of an element in a compound (EX: %C in CO)

(number of atoms of element in compound) X (average atomic mass of element)/ formula mass of the compound X 100% (on your answer after dividing the numbers)

%C in CO?

1 × 12.01 amu/ 28.01 = 0.42877 × 100% = 42.88% mass composition of Carbon. If you subtract 42.88% of 100%, your answer 57.12% is the percent composition of Oxygen. (you can also verify your work by checking the periodic table. Oxygen has a higher amu than Carbon does, so this helps prove our answer right).

250g of a substance contains 38g of Oxygen. What is the % of O?

% of O = 38g/250g x 100 = 15%