Percentage Percentage Composition (page Composition (page 118)118)

The Law of Definite Composition deals The Law of Definite Composition deals with with COMPOUNDSCOMPOUNDS. This law states that . This law states that the composition of any pure compound is the composition of any pure compound is constant. The elements that make up a constant. The elements that make up a compound are always present in the compound are always present in the same ratio by mass.same ratio by mass.

Water is always 11.21% hydrogen by Water is always 11.21% hydrogen by mass and 88.79% oxygen by mass.mass and 88.79% oxygen by mass.

Sodium chloride is always 39.34% Sodium chloride is always 39.34% sodium by mass and 60.66% chlorine by sodium by mass and 60.66% chlorine by mass.mass.

This is true regardless of the size of the This is true regardless of the size of the sample or where the sample comes from.sample or where the sample comes from.

To calculate the percent composition To calculate the percent composition (percentage composition) of a compound (percentage composition) of a compound

Calculate the Calculate the molecular massmolecular mass (molecular (molecular weight, formula mass, formula weight), weight, formula mass, formula weight), MM, of the compound MM, of the compound

Calculate the total mass of each element Calculate the total mass of each element present in the formula of the compoundpresent in the formula of the compound

Calculate the percent compositon     = Calculate the percent compositon     = (total mass of element present ÷ molecular mass) x 100 (total mass of element present ÷ molecular mass) x 100

In general the percent of In general the percent of element A in compound AB element A in compound AB can be represented bycan be represented by

%A = %A = mass of A mass of A X 100% X 100%

mass of ABmass of AB

Determine the Determine the percentage composition percentage composition of NaCl.of NaCl.

What is the percentage What is the percentage composition of Nacomposition of Na22SOSO44??

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Complete the handout givenComplete the handout given Complete question 7 on page 119Complete question 7 on page 119