1.3 Elemental Composition of Pure Substances

A pure substance is one with ______ composition

constant

A pure substance can either be a(n):

element or compound

Compounds with the same elements in the same proportion are the same compound

Law of definite proportion

When dealing with compounds you can assume it follows the:

law of definite proportion

Following the law of definite proportion, you can find the:

percent composition

Percent by mass of each element that makes up a compound

Percent composition

How to calculate percent composition?

divide the mass of each element in a compound by the total molar mass of the substance.

The simplest ratio of one element to another in a compound

Empirical formula

the actual formula for the substance

Molecular formula

How to determine empirical formula when given percent of each element?

a. Assume you are given a 100g sample so you can change percent to grams

b. For each element take grams / molar mass to get moles of each element

c. Divide each mole value by the lowest of the values

d. If you are within 0.1 of a whole number round to the whole number, if you are not you must multiply by a factor that gives you whole numbers for all.

e. The values you found are the subscripts for each element

How to determine molecular formula?

a. Find mass of empirical formula

b. Molar mass/ empirical formula mass to find factor

c. Multiply all subscripts in the empirical formula by the value