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Molar Conversion Quiz

Percent Composition and Chemical Formulas

Agenda: 1. Review Percent Composition (LPA)

2. Intro to Empirical and Molecular Formulas3. Guided Practice

4. Independent Practice

Objective:

My words: SWBAT use percent composition of compounds to formulate empirical and molecular formulas of compounds as demonstrated through a handout/worksheet.

Review Percent Composition…

What are the two ways we can find the percent composition by mass of a compound?

If I have a 23 g sample of sodium chloride and found 14 g of that sample to be chlorine, what is the percent composition of the sample?

Theoretically, what should the percent composition of sodium chloride be?

Percent Composition as a conversion factor…

You can use percent composition to find the number of grams of any element in a specific compound!

Multiply the mass of the compound by a conversion factor based on the percent composition of that element to the compound!

Percent composition as a conversion factor…

Example:

Suppose you want to know how much carbon and hydrogen are contained in 90 g of propane (C3H8).

1. Find % composition2. Use that as conversion factor3. Solve for the element’s mass

Entrance Ticket

You have a sample of table salt in front of you and find that it was 60% chlorine and 40% sodium by mass. If the total mass of your sample was determined to be 50 g of table salt, how much of that mass is chlorine and how much of that mass is sodium?

Empirical Formulas

The percent composition that you found is the data you will need to calculate the basic ratio of elements in that compound.

Basic ratio = empirical formula Empirical formula is the lowest whole-

number ratio of the atoms in a compound. Ionic formulas, as we learned, were always

lowest ratio possible, but molecular formulas were not always like this…

An empirical formula may or may not be the same as a molecular formula (true formula) of the compound

The molecular formula tells the actual amount and each kind of atoms present in the compound

Example: Molecular formula: H2O2

Empirical formula: HO

What are the empirical formulas for the following molecular formulas:

H2O

C2H4

C3H8

C2H4O2

Determining the Empirical formula when you do not know the molecular formula

Sometimes you do not know the molecular formula (such as an unknown compound in front of you)

Steps:

1. Find the percent composition2. Find the moles of each element3. Divide by the lowest moles

present and get to a whole number.

Sample Problem 1

A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula for the compound?

Sample Problem 2

Calculate the empirical formula for a compound that contains 94.1% oxygen and 5.9% hydrogen

Sample Problem 3

Calculate the empirical formula for a compound that is 67.6% Hg, 10.8% S, and 21.6% O.

Sample Problem 4

1,6-Diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62.1% C, 13.8% H and 24.1% N?

Determining Molecular Formula

Determining the molecular formula is simple when you know your empirical formula!

The molecular formula is either the same as the empirical formula or a simple whole-number multiple of the empirical formula

Steps:1. Find empirical formula2. Divide Molar Mass by EFM3. multiply through your EF by the

whole number found in step 2

Sample Problem 5

Calculate the molecular formula of a compound whose molar mass is 60 g/mol and empirical formula was found to be CH4N.

Sample Problem 6

Find the molecular formula of ethylene glycol, which is used in antifreeze. The molar mass is known to be 62 g/mol and the empirical formula is CH3O.

Which pair of molecules has the same empirical formula?

a. C2H4O2; C6H12O6

b. NaCrO4; Na2Cr2O7

Recap Exit SlipHow is the empirical

formula different from molecular formula?

How do we find the empirical formula?

How do we find the molecular formula?