Empirical Formulas 4/1 - ATB: If an unknown sample has 1 mole of H and 1 mole of Cl, what would be the formula of this compound?

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Presentation transcript:

Empirical Formulas 4/1 - ATB: If an unknown sample has 1 mole of H and 1 mole of Cl, what would be the formula of this compound?

Empirical Formulas We have already determined the percent composition by mass of a compound from a chemical formula and from experimental data.

Empirical Formulas We have already determined the percent composition by mass of a compound from a chemical formula and from experimental data. Now we will head the opposite direction!

Chemical Formulas of Compounds Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio

Chemical Formulas of Compounds Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio H 2 O: H 2 O:

Chemical Formulas of Compounds Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio H 2 O: H 2 O:  2 atoms of H for every 1 atom of O

Chemical Formulas of Compounds Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio H 2 O: H 2 O:  2 atoms of H for every 1 atom of O  OR 2 moles of H for every 1 mole of O

Chemical Formulas (continued)‏ If we know or can determine the relative number of moles of each element in a compound, we can determine a formula for the compound.

Types of Chemical Formulas Empirical Formula:

Types of Chemical Formulas Empirical Formula:  The formula of a compound that expresses the smallest whole number ratio of the atoms present

Types of Chemical Formulas Empirical Formula:  The formula of a compound that expresses the smallest whole number ratio of the atoms present  Ionic formulas are always empirical formulas

Types of Chemical Formulas Empirical Formula:  The formula of a compound that expresses the smallest whole number ratio of the atoms present  Ionic formulas are always empirical formulas Molecular Formula:

Types of Chemical Formulas Empirical Formula:  The formula of a compound that expresses the smallest whole number ratio of the atoms present  Ionic formulas are always empirical formulas Molecular Formula:  The formula that states the actual number of each kind of atom found in one molecule of the compound.

To Obtain an Empirical Formula 1. Determine the mass (in g) of each element. (may already be done for you!)‏

To Obtain an Empirical Formula 1. Determine the mass (in g) of each element. (may already be done for you!)‏ 2. Calculate the number of moles of each element

To Obtain an Empirical Formula Determine the mass (in g) of each element. (may already be done for you!)‏ Calculate the number of moles of each element Divide each by the smallest number of moles to obtain the simplest whole number ratio

Lets Give it a Whirl! A sample of a brown gas is found to contain 2.34g of N and 5.34g of O. Determine the empirical formula of the substance.

Lets Give it a Whirl! A sample of a brown gas is found to contain 2.34g of N and 5.34g of O. Determine the empirical formula of the substance.  Step 1: Already done!  Step 2: Find the number of moles of N and O  Step 3: Divide by the smallest number of moles.

Another Example: A substance is found to be 5.88% hydrogen and 94.12% oxygen. What is the empirical formula?

Another Example: A substance is found to be 5.88% hydrogen and 94.12% oxygen. What is the empirical formula?  Step 1: Determine the number of g of H and O We can “fudge” this!  Step 2: Find the number of moles of H and O  Step 3: Divide by the smallest number of moles.

Molecular Formulas 4/3 ATB: Ribose has a molar mass of 150g/mol and a chemical composition of 40.0% carbon, 6.67% hydrogen, and 53.3% oxygen. What is the empirical formula of ribose?

Molecular Formulas 3/2 ATB: Ribose has a molar mass of 150g/mol and a chemical composition of 40.0% carbon, 6.67% hydrogen, and 53.3% oxygen. What is the empirical formula of ribose? Empirical Formula: CH 2 O

Determining Molecular Formulas What is the molecular formula when compared to the empirical formula?

Determining Molecular Formulas What is the molecular formula when compared to the empirical formula?  It contains the actual number of atoms of each element found in a compound.

Determining Molecular Formula

1 st : Find the Empirical Formula (1-2-3)‏

Determining Molecular Formula 1 st : Find the Empirical Formula (1-2-3)‏ 2 nd : Find the molar mass of the empirical formula

Determining Molecular Formula 1 st : Find the Empirical Formula (1-2-3)‏ 2 nd : Find the molar mass of the empirical formula 3 rd : Divide the molar mass of the compound by the molar mass of the empirical formula – that is your multiplication factor!

Time to Practice! Determine the molecular formula of ribose.

Time to Practice! Determine the molecular formula of ribose.  1. Determine the empirical formula (already done!)‏

Time to Practice! Determine the molecular formula of ribose.  1. Determine the empirical formula (already done!)‏  2. Determine the molar mass of CH 2 O (the empirical formula)‏

Time to Practice! Determine the molecular formula of ribose.  1. Determine the empirical formula (already done!)‏  2. Determine the molar mass of CH 2 O (the empirical formula)‏  3. Divide the molar mass of the compound by the molar mass of the empirical formula.

Lets Try Another! The explosive, TNT, is composed of 37.0% carbon, 2.20% hydrogen, 18.5% nitrogen, and 42.3% oxygen. If the molar mass of TNT is 227g/mol, what is the molecular formula?

Lets Try Another! The explosive, TNT, is composed of 37.0% carbon, 2.20% hydrogen, 18.5% nitrogen, and 42.3% oxygen. If the molar mass of TNT is 227g/mol, what is the molecular formula?  1. Find the empirical formula

Lets Try Another! The explosive, TNT, is composed of 37.0% carbon, 2.20% hydrogen, 18.5% nitrogen, and 42.3% oxygen. If the molar mass of TNT is 227g/mol, what is the molecular formula?  1. Find the empirical formula  2. Find the molar mass of the empirical formula.

Lets Try Another! The explosive, TNT, is composed of 37.0% carbon, 2.20% hydrogen, 18.5% nitrogen, and 42.3% oxygen. If the molar mass of TNT is 227g/mol, what is the molecular formula?  1. Find the empirical formula  2. Find the molar mass of the empirical formula.  3. Divide the molar mass of TNT by the molar mass of the empirical formula.