Chapter 7 – The Mole and Chemical Composition Sec 3 - Formulas and Percentage Composition...

Chapter 7 – The Mole and Chemical Composition

Sec 3 - Formulas and Percentage Composition

Definitions:

Percent Composition: the percentage by mass of each elementin a compound

Empirical Formula a chemical formula that shows the compositionof a compound in terms of the relative numbers and kinds of atoms inthe simplest ratio

Molecular Formula a chemical formula that shows the numberand kinds of atoms in a molecule, but not the arrangement of the atoms



Percent Composition


Sec 3 - Formulas and Percentage CompositionHow to calculate the percent composition of a compound.

1. Calculate the mass of each element in the compound. For example if the compound contains 3 oxygen atoms the mass of the oxygen in the compound is 16.0 x 3 = 48

2. Calculate the gram molar mass of the whole compound. For example if the compound was CaSO3, the total mass would be 40.1 from calcium + 32.0 from sulfur and 48.0 from oxygen. The total would be 40.1 + 32.0 + 48.0 for A total mass of 120.1.

3. Follow the formula on your Reference Table:

Mass of the part is step 1, mass of the whole is step 2.


Sec 3 - Formulas and Percentage CompositionPractice you percent composition skills

1. Calculate the percent composition of sodium chloride, NaCl

2. Calculate the percent composition of silver nitrate,AgNO3

3. Calculate the percent composition of magnesium hydroxide, Mg(OH)2

4. What is the mass percentage of water in the hydrate CuSO4• 5 H2O

Na – 39.7% Cl- 60.3 %

Ag - 63.5% N - 8.2% O - 28.3%

Mg – 41.7% O – 54.9% H - 3.4%

Water is 36.1% of the compound



Homework Page 860 #47, #48 and #49



Difference between empirical formula and molecular formula

Empirical Formula: Simplest ratio of elements

Molecular Formula: “True formula” Does not have to be simplest ratio

C6H12O6 CH2OReduced by factor of 6

Molecular Empirical

H2OBoth empirical and molecular



Calculating Empirical Formula

1. Set up the problem by each element

2. divide each element quantity by its gram molar mass from the periodic table

3. Look at each answer and divide the smallest answer into each elements answer.

4. This will produce the ratio of each element in the empirical formula

5. If ratio is not whole numbers increase all answers by a factor to create whole numbers

6. Write empirical formula



A compound is found to contain 54.5% carbon, 9.1% hydrogen, and 36.4% oxygen. Determine the simplest formula.

Carbon Hydrogen Oxygen

54.5 9.1 36.412 1.0 16.0

4.54 9.1 2.272.27 2.27 2.27

2 4 1

C2H4O



Find the empirical formula of a compound found to contain 26.56% potassium, 35.41% chromium, and the remainder oxygen.

Potassium Chromium Oxygen

26.56 35.41 38.0339.1 52.0 16.0

0.68 0.68 2.340.68 0.68 0.68

1.0 1.0 3.5

Increase by factor of 2 to make all whole numbers

K2Cr2O7



Homework page 861 #69 - #73


Sec 3 - Formulas and Percentage CompositionCalculating Molecular Formula

1. Calculate the empirical formula if not given.

2. Calculate the empirical formulas gram formula mass

3. Divide the molecular formulas gram formula mass by the molecular formulas gram formula mass. This will give you a factor number that you must increase the ratio of the empirical formula by.



Determine the molecular formula of a compound with an empirical formula of NH2 and a formula mass of 32.06 amu.

NH2 gram formula mass is 16.0 amu

Molecular formulas gram formula mass is 32.06 amu

32.0616.0 = 2

You must increase the ratio of NH2 by a factor of 2

N2H4









Date post:	21-Dec-2015
Category:	Documents
Upload:	cecil-clarke
View:	233 times
Download:	7 times

Chapter 7 – The Mole and Chemical Composition Sec 3 - Formulas and Percentage Composition...

Documents