an unknown compound contains 85.64% carbon with the remainder hydrogen. It has a molar mass of 42.08 g/mol. Find its empirical and molecular formulas.
I think I did this yesterday.
Assume 100 grams comound
MolesC=85.64/12= 7.14
MolesH=14.36/1=14.36
now divide both moles by the smaller of the two (7.14) so in the end, the ratio is CH2.
But the molmass is 42, so take the mole mass of CH2 (14), divide it into 42, giving 3. The molecular formula is C3H6
To determine the empirical and molecular formulas of the unknown compound, we need to follow a series of steps. Let's start by finding the empirical formula:
Step 1: Find the percentage composition of each element.
In this case, we know that the compound contains 85.64% carbon and the remainder is hydrogen. Therefore, we can assume that the compound contains 100% - 85.64% = 14.36% hydrogen.
Step 2: Convert the percentage composition to moles.
To convert percentages to moles, we assume a 100g sample of the compound. Using the atomic masses of carbon (12.01 g/mol) and hydrogen (1.008 g/mol), we can calculate the moles of each element.
- Carbon: (85.64 g / 12.01 g/mol) = 7.13 mol
- Hydrogen: (14.36 g / 1.008 g/mol) = 14.25 mol
Step 3: Divide the number of moles by the smallest number of moles.
To determine the simplest, or empirical, formula, we divide the number of moles of each element by the smallest number of moles. In this case, the smallest number of moles is 7.13 mol.
- Carbon: 7.13 mol / 7.13 mol = 1
- Hydrogen: 14.25 mol / 7.13 mol = 2
Therefore, the empirical formula is CH2.
Now, let's move on to finding the molecular formula:
Step 4: Determine the molar mass of the empirical formula.
To calculate the molar mass of the empirical formula (CH2), we use the atomic masses of carbon and hydrogen.
Molar mass of carbon = 12.01 g/mol
Molar mass of hydrogen = 1.008 g/mol
Molar mass of CH2 = (1 * 12.01 g/mol) + (2 * 1.008 g/mol) = 14.03 g/mol
Step 5: Divide the molar mass of the compound by the molar mass of the empirical formula.
To find the molecular formula, we divide the molar mass of the compound by the molar mass of the empirical formula.
Molar mass of the compound = 42.08 g/mol
42.08 g/mol / 14.03 g/mol = ~3
Since the result is approximately 3, we multiply the empirical formula (CH2) by the factor of 3:
CH2 * 3 = C3H6
Therefore, the molecular formula of the unknown compound is C3H6.