Derive the empirical formula of a hydrocarbon that on analysis gave the following percentage by mass composition: C = 85.63 % and H = 14.37 %. If the molecular mass of this compound is 56 g, derive the molecular formula.
determine the mass in grams by treating the percentage values as masses (by mass composition)
calculate the mole of each element by mass/molar mass
divide each mole with the lowest mole and round off to whole number, to give you the empirical formula
to determine the molecular formula, just divide the molecular mass by the empirical mass.
hope that helps!!
To derive the empirical formula of a compound, you need to determine the simplest whole number ratio of the elements present in the compound.
Step 1: Convert the percentage by mass to grams.
- If you have 100 g of the compound, then you would have 85.63 g of carbon (C) and 14.37 g of hydrogen (H).
Step 2: Determine the number of moles of each element.
- To find the number of moles, divide the mass of each element by its molar mass, which is the atomic mass found on the periodic table.
- The molar mass of carbon (C) is approximately 12 g/mol.
- The molar mass of hydrogen (H) is approximately 1 g/mol.
- Therefore, the number of moles of carbon (nC) can be calculated by dividing the mass of carbon by its molar mass: nC = 85.63 g / 12 g/mol = 7.14 mol.
- Similarly, the number of moles of hydrogen (nH) can be calculated as: nH = 14.37 g / 1 g/mol = 14.37 mol.
Step 3: Find the simplest whole number ratio.
- Divide each of the moles by the smaller number of moles.
- In this case, the number of moles of carbon is smaller, so we divide each of the moles by 7.14 mol.
- nC / 7.14 mol = 1
- nH / 7.14 mol = 2
Step 4: Round the ratios to the nearest whole number.
- The simplest whole number ratio is C1H2.
Now, let's move on to determining the molecular formula.
Step 5: Calculate the empirical formula mass.
- The empirical formula mass is the sum of the atomic masses in the empirical formula.
- In this case, the empirical formula mass is: 1(12 g/mol) + 2(1 g/mol) = 14 g/mol.
Step 6: Calculate the ratio of the molecular mass to the empirical formula mass.
- The ratio is calculated as: molecular mass / empirical formula mass = 56 g/mol / 14 g/mol = 4.
Step 7: Multiply the subscripts in the empirical formula by the ratio obtained in Step 6.
- The molecular formula is obtained by multiplying the subscripts in the empirical formula by the ratio.
- In this case, the molecular formula is: C1H2(4) = C4H8.
Therefore, the empirical formula for the hydrocarbon is C1H2, and the molecular formula is C4H8.