empirical and molecular

Chapter EightChapter Eight

Part II

Obtain one piece of gum, and find its mass in the wrapper.

Keep the wrapper for later and start chewing. We are experimenting.

Percents

If you have 7 green cars, 8 blue cars, and 4 red cars, what percentage is green?

Percents

% = part

wholex 100

Percents

The mass of a mini Hershey Bar is 6.5 grams and the mass of a mini Mr. Goodbar is 8.3 grams. If you have a basket with 5 Hershey bars and 3 Mr. Goodbars, what is the percent BY MASS of each?

Percentage Composition

Percent Composition is finding the percent by mass of each element in a compound using the periodic table.

Percentage Composition

What is the percent by mass of each element in FeSO4?

Percentage Composition

What is the percent by mass of each element in Mg3(PO4)2?

Percentage Composition

What is the percent by mass of carbon in sucrose, C12H22O11?

Percentage Composition

An 8.20 gram piece of magnesium combines with 5.40 grams of oxygen to form a compound.

What is the percent composition of this compound?

Review

Find the percent of oxygen in each of the following:

Rb2O

Al(NO3)3

Percentage Composition

How many grams of Ca are in 64.3 grams CaCl2?

You Try

How many grams of Oxygen are in 19.2 grams Na3PO4?

You try

How many grams of oxygen are in 56.0 grams of each of the following:

Rb2O

Al(NO3)3

Finding a ratio

If a school has 30 boys and 60 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

If a school has 27 boys and 18 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

If a school has 75 boys and 60 girls, what is the ratio of boys to girls?

Finding a whole number ratio in its lowest terms

A certain compound contains 4.14 moles of Mg and 2.76 moles of P.

Find the mole ratio between Mg and P.

Write the formula.

Empirical Formula

An Empirical formula gives the lowest whole number MOLE ratio between the atoms in a compound.

The Math

A compound contains 0.900 grams of calcium and 1.60 grams of chlorine.

Find its empirical formula.

The Math

A compound is analyzed and found to consist of 36.0 % aluminum and 64.2 % sulfur.

Find its empirical formula.

We Try

Calculate the empirical formula of a compound that is 62.0 % C, 13.9 % H, and 24.1 % N.

You Try

What is the empirical formula of a compound that is 66.0 % Ca and 34.0 % P?

You Try

Benzoic Acid is used as a preservative, A 15.0 gram sample contains 10.32 grams of C, 0.743 grams of H, and 3.93 grams of O. Find the empirical formula.

Molecular Formula

A Molecular formula gives the actual ratio between the atoms in a compound.

For Ionic compounds, the empirical and molecular formulas are the same.

Molecular Formula

For molecular compounds, the molecular formula is a multiple of the empirical.

Example:C6H6 is benzene. What is

benzene’s empirical formula?

Molecular Formula

Example:C2H6 is ethane. What is

ethane’s empirical formula?

The Math

A compound contains 31.10 grams of carbon and 7.872 grams of hydrogen.

Find its empirical formula.

The Math

What is the same compound’s molecular formula if its molar mass = 90.18 g/mol?

Molecular formula

What is the molecular formula of dichloroacetic acid, if the empirical formula is CHOCl, and its molar mass is 129 g/mol.

Molecular formula

Vitamin C is ascorbic acid. It has a percentage composition of 40.9 % carbon, 4.58 % hydrogen, and 54.5 % oxygen with a molecular mass of 176.1 u.

Find its empirical and molecular formulae.

Review Molecular formula

Aspirin, acetylsalicylic acid, contains 60.0 % carbon, 4.48 % hydrogen, and 35.5 % oxygen with a molecular mass of 180 u.

Find its empirical and molecular formulae.

Molecular formula

Find its empirical and molecular formulae, if a compound contains 40.0 % carbon, 6.73 % hydrogen, and 53.3 % oxygen with a mass of 240 g/mol.

Hydrates

Hydrates are compounds with water molecules trapped in their crystal structure.

The water is weakly bonded and can be removed by heating the substance.

Hydrates

Example:Li2SO4 * 2 H2O

This formula tells you that 1 mole of lithium sulfate is bonded to 2 moles of water.

Hydrates

Example:MgCO3 * 7 H2O

This formula tells you that 1 mole of magnesium carbonate is bonded to 7 moles of water.

Hydrates

Example:MgSO3 * 6 H2O

This formula tells you that 1 mole of magnesium sulfite is bonded to 6 moles of water.

Hydrates

Notice the mole ratio is shown as a coefficient and the compound gets the 1.

General form: any compound * __ H2O

We try

0.391 grams of Li2SiF6 is found to contain 0.0903 grams of water.

Find the formula for the hydrate

We try

A compound is analyzed and found to contain 86.7 % Mo2S5 and 13.3 % water.

Find the formula for the hydrate

You try

A particular hydrate contains 89.2 % BaBr2.

Find the formula for the hydrate.

Quiz Review

A particular hydrate of MgSO4 contains 51.14 % water.

Find the formula for the hydrate.

Quiz Review

Find the percentage of water in the hydrate, Na2CO3 * 10 H2O.

Quiz Review

Find the percentage of oxygen in P2O5.

How many grams of oxygen are in 25.5 grams of P2O5?