Molecular Formula: The actual formula for a molecule.Problem:A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular weight of 318.31 g/mol. What is the molecular formula for this compound? Show
Strategy:
The empirical formula of a chemical compound is a representation of the simplest whole number ratio between the elements comprising the compound. The molecular formula is the representation of the actual whole number ratio between the elements of the compound. This step-by-step tutorial shows how to calculate the empirical and molecular formulas for a compound. Empirical and Molecular ProblemA molecule with a molecular weight of 180.18 g/mol is analyzed and found to contain 40.00% carbon, 6.72% hydrogen and 53.28% oxygen. How to Find the SolutionFinding the empirical and molecular formula is basically the reverse process used to calculate mass percent or mass percentage. Step 1: Find the number of moles of each element in a sample of the molecule. 40.00 grams
of carbon (40.00% of 100 grams) Note: 100 grams is used for a sample size just to make the math easier. Any sample size could be used, the ratios between the elements will remain the same. Using these numbers, we can find the number of moles of each element in the 100-gram sample. Divide the number of grams of each element in the sample by the atomic weight of the element to find the number of moles. moles C = 40.00 g x 1 mol C/12.01 g/mol C = 3.33 moles C moles H = 6.72 g x 1 mol H/1.01 g/mol H = 6.65 moles H moles O = 53.28 g x 1 mol O/16.00 g/mol O = 3.33 moles O Step 2: Find the ratios between the number of moles of each element. Select the element with the largest number of moles in the sample. In this case, the 6.65 moles of hydrogen is the largest. Divide the number of moles of each element by the largest number. Simplest mole ratio between C and H: 3.33 mol C/6.65 mol H = 1 mol C/2 mol H The simplest ratio between O and H: 3.33 moles O/6.65 moles H = 1 mol O/2 mol H Step 3: Find the empirical formula. We have all the information we need to write the empirical formula. For every two moles of hydrogen, there is one mole of carbon and one mole of oxygen. The empirical formula is CH2O. Step 4: Find the molecular weight of the empirical formula. We can use the empirical formula to find the molecular formula using the molecular weight of the compound and the molecular weight of the empirical formula. The empirical formula is CH2O. The molecular weight is molecular weight of CH2O = (1 x 12.01 g/mol) + (2 x 1.01 g/mol) + (1 x 16.00 g/mol) Step 5: Find the number of empirical formula units in the molecular formula. The molecular formula is a multiple of the empirical formula. We were given the molecular weight of the molecule, 180.18 g/mol. Divide this number by the molecular weight of the empirical formula to find the number of empirical formula units that make up the compound. Number of empirical formula units in compound = 180.18 g/mol/30.03 g/mol Step 6: Find the molecular formula. It takes six empirical formula units to make the compound, so multiply each number in the empirical formula by 6. molecular formula = 6 x CH2O Solution: The empirical formula of the molecule is CH2O. Limitations of the Molecular and Empirical FormulasBoth types of chemical formulas yield useful information. The empirical formula tells us the ratio between atoms of the elements, which can indicate the type of molecule (a carbohydrate, in the example). The molecular formula lists the numbers of each type of element and can be used in writing and balancing chemical equations. However, neither formula indicates the arrangement of atoms in a molecule. For example, the molecule in this example, C6H12O6, could be glucose, fructose, galactose, or another simple sugar. More information than the formulas is needed to identify the name and structure of the molecule. Empirical and Molecular Formula Key Takeaways
How do you find the molecular and empirical formula?Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number. Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.
How do you calculate the empirical formula?Calculate the empirical formula.. In any empirical formula problem you must first find the mass % of the elements in the compound. ... . Then change the % to grams. ... . Next, divide all the masses by their respective molar masses. ... . Pick the smallest answer of moles and divide all figures by that.. What are the steps to calculate a molecular formula?The steps involved in determining a Molecular Formula are: Find the mass of the empirical formula. Divide the molecular mass by the mass of the empirical formula. Multiply each subscript in the empirical formula by the answer in step 2.
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