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Empirical Formula and Molecular Formula. Empirical Formula  Empirical formula is the simplest whole number ratio of atoms in a formula  Empirical information.

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Presentation on theme: "Empirical Formula and Molecular Formula. Empirical Formula  Empirical formula is the simplest whole number ratio of atoms in a formula  Empirical information."— Presentation transcript:

1 Empirical Formula and Molecular Formula

2 Empirical Formula  Empirical formula is the simplest whole number ratio of atoms in a formula  Empirical information is always based on lab data

3 Molecular Formula  Molecular formula is the actual ratio of the atoms that form a molecule of a substance.  Sometimes the empirical and molecular are the same but they do not have to be!

4 More molecular….  Example H 2 O for water empirical and molecular formula are the same.  H 2 : O 1 is the simplest whole number ratio of atoms in the formula and it is also how the molecule actually forms.  Example glucose C 6 H 12 O 6 for glucose the molecule is 6:12:6 This is not the simplest whole number ratio of atoms in the formula.

5 Molecular and Empirical Formulas  If glucose were analyzed in the laboratory its empirical ratio would be 1:2:1. Additional test would have to be run to determine the mass of the substance

6 Calculations Step one= List individual elements in the formula. Change the given information from grams or percent into moles Step two= Decide which mole value is the smallest on the list. Step three= Divide all mole values by that smallest value

7 more Calculations……  Step four= If a decimal of.5 or.3 appear in the ratio you obtained do not round up or off! Ratios with.5 must be multiplied by two. Ratios with.3 must be multiplied by three.

8 Formula of Hydrates  The three basic steps are the same for hydrates. But with hydrates……  The two parts being compared are:  Moles of anhydrous in the sample vs.  Moles of water in the sample  Elements are not listed separately!

9 Example Empirical /Molecular  An organic compound was found to contain 92.25 % carbon and 7.75 % hydrogen. If the molecular mass is 78.0 grams, please calculate both the empirical and molecular formulas for this substance.

10 Step 1  C 92.25g/12.01 g = 7.681 moles  H 7.75g/1.01 g= 7.67 moles  Divide through by the smaller value!  1 : 1 ratio C 1 H 1 is empirical formula  Now we must check for molecular formula!

11 Step 2  C 1 H 1 add up the empirical mass of this substance.  12.01+ 1.01g = 13.02grams  Divide the given molecular mass by the empirical mass.  78.0/ 13.02 = 6  Molecular formula is C 6 H 6

12 Final Answer  Empirical formula C 1 H 1  Molecular formula C 6 H 6

13 Modern Chemistry example p.246  Quantitative analysis shows that a compound contains 32.38% sodium, 22.65%sulfur, and 44.99% oxygen. Please find the empirical formula for this compound.

14 Hydrate Example  Strontium chloride hexahydrate  158.52g plus 108.12g = 266.64g  108.12/266.64 x 100 = 40.5 % H 2 O

15 Hydrate Calculation  CuSO 4 * ? H 2 O  Crucible&cover111.55g  Crucible& cov& hydrate131.53g  Crucible/cov& anhydrous124.32g

16 Hydrates  Subtract C&C from anhydrous (after heating) this will equal grams of CuSO4.  Change grams to moles.

17 Hydrates  If you subtract hydrate from anhydrous you will find the mass of the water removed from the sample.  Change grams water to moles.

18 Hydrates  Look at mole values.  Decide which mole value is smaller  Divide both values by the smaller to get a one to something ratio  Taa! Daa! you are done.

19 The End!  Go forth and calculate!


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