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**From percentage to formula**

Empirical Formula From percentage to formula

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**Types of Formulas Empirical Molecular (true) Name CH C2H2 acetylene**

The formulas for compounds can be expressed as an empirical formula and as a molecular(true) formula. Empirical Molecular (true) Name CH C2H2 acetylene CH C6H6 benzene CO2 CO2 carbon dioxide CH2O C5H10O5 ribose

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**An empirical formula represents the simplest whole number ratio of the atoms in a compound.**

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**It is not just the ratio of atoms, it is also the ratio of moles of atoms**

In 1 mole of CO2 there is 1 mole of carbon and 2 moles of oxygen In one molecule of CO2 there is 1 atom of C and 2 atoms of O

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**The molecular formula is the true or actual ratio of the atoms in a compound.**

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**Learning Check EF-1 1) CH2O 2) C2H4O2 3) C3H6O3**

A. What is the empirical formula for C4H8? 1) C2H ) CH ) CH B. What is the empirical formula for C8H14? 1) C4H7 2) C6H ) C8H14 C. What is a molecular formula for CH2O? 1) CH2O 2) C2H4O ) C3H6O3

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**Learning Check EF-1 1) CH2O 2) C2H4O2 3) C3H6O3**

A. What is the empirical formula for C4H8? 1) C2H ) CH ) CH B. What is the empirical formula for C8H14? 1) C4H7 2) C6H ) C8H14 C. What is a molecular formula for CH2O? 1) CH2O 2) C2H4O ) C3H6O3

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Learning Check EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 1) SN 2) SN4 3) S4N4

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Learning Check EF-2 If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain. 3) S4N4

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**Empirical and Molecular Formulas**

molar mass = a whole number = n simplest mass n = 1 molar mass = empirical mass molecular formula = empirical formula n = 2 molar mass = 2 x empirical mass molecular formula = 2 x empirical formula molecular formula = or > empirical formula

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**Learning Check EF-3 2) C6H8O6 3) C9H12O9**

A compound has a formula mass of and an empirical formula of C3H4O3. What is the molecular formula? 1) C3H4O3 2) C6H8O6 3) C9H12O9

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**Learning Check EF-3 2) C6H8O6 3) C9H12O9**

A compound has a formula mass of and an empirical formula of C3H4O3. What is the molecular formula? 1) C3H4O3 2) C6H8O6 3) C9H12O9

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**Learning Check EF-4 1) C7H6O4 2) C14H12O8 3) C21H18O12**

If there are g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 1) C7H6O4 2) C14H12O8 3) C21H18O12

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**Learning Check EF-4 1) C7H6O4 2) C14H12O8 3) C21H18O12**

If there are g of O in the molecular formula, what is the true formula if the EF is C7H6O4? 1) C7H6O4 2) C14H12O8 3) C21H18O12

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**Calculating Empirical Formula**

Pretend that you have a 100 gram sample of the compound. That is, change the % to grams. Convert the grams to mols for each element. Write the number of mols as a subscript in a chemical formula. Divide each number by the least number. Multiply the result to get rid of any fractions.

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Example Calculate the empirical formula of a compound composed of % C, % H, and %N. Assume 100 g so 38.67 g C x 1mol C = mole C gC 16.22 g H x 1mol H = mole H gH 45.11 g N x 1mol N = mole N gN

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**If we divide all of these by the smallest**

one it will give us the subscripts for the empirical formula 3.220 mol C = 1 3.219 mol N 16.09 mol H = 5 3.219 mole N = 1 Empirical formula: CH5N

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Learning Check EF-5 Aspirin is 60.0% C, 4.5 % H and 35.5 O. Calculate its simplest formula. In 100 g of aspirin, there are g C, 4.5 g H, and g O.

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Solution EF-5 60.0 g C x ___________= ______ mol C 4.5 g H x ___________ = _______mol H 35.5 g O x ___________ = _______mol O

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**5.00 mol C = ________________ ______ mol O **

Divide by the smallest # of moles. 5.00 mol C = ________________ ______ mol O 4.5 mol H = ________________ 2.22 mol O = ________________ Are are the results whole numbers?_____

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Finding Subscripts A fraction between 0.1 and 0.9 must not be rounded. Multiply all results by an integer to give whole numbers for subscripts. (1/2) 0.5 x 2 = 1 (1/3) x 3 = 1 (1/4) 0.25 x 4 = 1 (3/4) 0.75 x 4 = 3 (1/5) 0.20 x 5 = 5

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Homework Worksheet C: #1-7

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