Empirical Formula vs. Molecular Formula. Chemical Formula Gives the combining whole number ratios of...
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Transcript of Empirical Formula vs. Molecular Formula. Chemical Formula Gives the combining whole number ratios of...
Empirical Formula vs. Molecular Formula
Chemical Formula
• Gives the combining whole number ratios of the elements in a compound
• C6H12O6
Structural Formula
• Gives the spatial arrangement of atoms in the compound
• Structural formula for H2O is H – O – H
Empirical Formula
• Only gives the types of elements in the compound and the ratio of the elements in the formula
Empirical Formula
• Does not tell exactly how many of the elements are in the compound
Molecular Formula
• Gives you the exact elemental composition of the compound
• Formula of the compound as it would actually exist.
EF vs. MF
Sucrose or table sugar:
Molecular Formula = C6H12O6
Empirical Formula = CH2O
Sample Problem
• The compound adrenaline contains % C = 56.79 % H = 6.56 % O = 28.37 % N = 8.28 by mass. Find the empirical formula.
Empirical Formula
• EF Determination when % Masses are given
Steps to Solve for EF
• Step 1: Sum up all given percentages. If total equals 100%, go to step 2. If total does not equal 100, the missing % is due to one of the component elements.
• Step 2: Convert Mass % to grams.• Step 3: Calculate moles using mole =
gram/molar mass•
Empirical Formula
• Step 4. To get simplest ratios, divide the moles calculated by the smallest calculated mole. You must have a ration of 1 for at least one of the element. (Follow rule for rounding).
• Step 5. You now have the ratios or subscripts for the EF.
Rule of Rounding Molar Ratios
• Mole ratios can only be rounded to the nearest whole number if they are < 0.2 away from the nearest whole number. For ex: 1.95 = 2; 3.18 = 3 and 4. 13 = 4.
• If the mole ratio is > 0.2 away from the nearest whole number, multiply the mole ratio by a certain integer to get it close to the nearest whole number. For ex: 3.5 x “2” = 7; 6.33 x “3” = 18.99 = 19; 4.25 x “4” = 11.
Please Remember• If you have to multiply a mole ratio by an
integer to get close to a whole number, you MUST multiply all the other mole ratios by the same integer.
• “In short, what you do to one mole ratio, you also do to the rest.”
• The ratios give you the subscripts in the EF.
Steps To Determine the Molecular Formula
• Step 1. Now that you have the empirical formula, get the ratio of the “given” molar mass to the empirical formula mass. Ratio = Given Molar Mass
Empirical Formula Mass
* Round ratio to the nearest whole number.
• Please note that the Empirical formula Mass is the sum of the atomic masses of all the elements in the Empirical Formula.
• Step 2. Once the ratio has been determined, multiply all the subscripts in the empirical formula by the ratio. This gives you the Molecular Formula.
Sample Problem
• Caffeine, a stimulant found in coffee, contains 49.5 % C, 5.15% H, 28.9 % N, and 16.5 % O by mass. The molar mass of the compound is 195 g/mol. Determine the empirical and molecular formula of caffeine.
Sample Problem
• Ibuprofen, a headache remedy, contains 75.69 % C, 8.80% H, and 15.51 % O by mass. The molar mass of the compound is 206 g/mol. Determine the empirical and molecular formula of ibuprofen.