4.5 Percentage Composition

Percentage Composition

New substance produced

Determine chemical composition

Determine composition by mass

Convert mass to % = % composition

Use atomic & molar mass to determine - correct chemical formula

% composition of water

Water = H + O

Results reveal - 2.5 g H reacted = 22.5 g of water

mH = 2.5g mH2O = 22.5g mo = (22.5g – 2.5g) = 20.0g %H = 2.5g/22.5g x 100% = 11.1% %O = 20.0g/22.5g x 100% = 88.9%

Sample Problem

Sodium and chlorine when they are allowed to react - combine to form sodium chloride

In an experiment, 3.45 g of sodium metal reacted with 5.33 g of chlorine gas to give 8.78 g of sodium chloride

Calculate % composition by mass of sodium chloride.

Solution

mNa+ =

mCl- =

mNaCl =

%Na =

%Cl =

Percentage Composition Calculations from a Formula

Calculate contribution of each element to the total mass

1st - cal mass of each element + atoms

2nd - cal total mass of all elements in compound

3rd - divide mass of each element by total mass = % comp

Sample Problem Determine % composition of sodium

phosphate Na3PO4

Solution