Essential Questions: How do you find percent composition



Essential Questions: How do you find percent composition?

What is an empirical formula and how do you find it?

How is an empirical formula related to molecular formula?

1) Determine the percent composition for CaO.

2) Determine the percent composition for a sample of compound CaO contains 10.73g of Ca and 4.28 g of O. What is the percentage of Ca and O?

3) Did having the actual mass in grams change the percent composition of the compound? Compare your answer to #1 to #2.

4) How would you work backwards to find the formula of a compound if given the percentage each of each element and the total mass of the compound? For example, a compound has a molar mass of 76.02 g/mol and consists of 36.9% N and 63.1% O. What is the formula?

SHOW YOUR WORK!!!

5) How do you think you could work backwards to find the formula of a compound if only given the percent composition? For example, a compound has the following composition: 11.2% Hydrogen and 88.8% Oxygen.

SHOW ANY WORK – WRITE DOWN ANY IDEAS!!

6) What do the following 2 compounds have in common? How are they different?

Similarities CH2O C6H12O6 Differences

Empirical formula = A formula that gives the simplest whole-number ratio of atoms in a compound.

Molecular formula = The same as or a multiple of the empirical formula, and is based on the actual number of atoms of each type in the compound.

7) Which compound from above is the empirical and which is the molecular?

8) Do the empirical and molecular formulas have the same percent composition??

9) So…if you are told the molar mass of the molecular formula and given the percent composition of the formula – how could you find the molecular formula? For example, a molecular compound has a molar mass of 152.04 g/mol and the following percent composition: 36.9% N and 63.1% O. What is the molecular formula?

SHOW YOUR WORK OR WRITE DOWN YOUR THOUGHTS!

**Steps to take- provide by me! Example: Molecular molar mass = 92.02 g/mol

30.4% N, 69.6%O

10 ) Now, what if you’re told the empirical formula and the molar mass of the molecular formula…how could you find the molecular formula? For example, a molecular compound has a molar mass of 228.06 g/mol and the following empirical formula: N2O3. What is the molecular formula?

SHOW YOUR WORK OR WRITE DOWN YOUR THOUGHTS!

**Steps to take- provide by me! Example: Molecular molar mass = 138.03 g/mol

Empirical formula = NO2

Classwork/Homework – SHOW ALL YOUR WORK FOR CREDIT!!!

Complete the following % composition problems for Classwork / Homework:

**Find the % composition of each element in each of the following compounds:

1. H2S

2. (NH4)2C2O4

3. MgN, Mg = 9.03 g N = 3.48g

4. A compound contains the elements copper and chlorine in a ratio of 1 mole of copper : 2 moles of chlorine

a. What is the empirical formula?

b. What is the % composition of Cu?

c. What is the % composition of Cl?

Complete the following for Homework! Find the empirical formula from each set of given data.

1. 67% Hg, 10.8% S, 21.6% O

2. 62.1% C, 13.8% H, 24.1% N

3. 75% carbon, 25% Hydrogen

Complete the following for homework: Find the molecular formula from each set of given data.

1. An unknown compound is found to have an empirical formula of CH3O. The molar mass of a compound is 62 g/mol. What is the molecular formula of the compound.

2. The empirical formula of a compound is NO2. Its molecular mass is 92 g/mol. What is its molecular formula?

3. A compound is found to be 40.0% carbon, 6.7% hydrogen, and 53.5% oxygen. Its molecular mass is 60 g/mol. What is its molecular formula?

4. A compound is 64.9% carbon, 13.5% hydrogen, and 21.6% oxygen. Its molecular mass is 74 g/mol. What is its molecular formula?

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***Steps to take – provided by me!! Example: 30.4% N and 69.6% O

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