A compound with a formula mass of 42.08 amu is found to be 85.64% carbon and 14.36 % hydrogen by mass. find its molecular formula.
so whats the answer
To find the molecular formula, we need to determine the empirical formula first. The empirical formula gives the simplest whole number ratio of atoms present in a compound.
Given:
Formula mass = 42.08 amu
Percentage of carbon (C) = 85.64%
Percentage of hydrogen (H) = 14.36%
1. Convert the percentages to grams:
Assume 100g of the compound.
Mass of carbon = 85.64g
Mass of hydrogen = 14.36g
2. Convert grams to moles:
To convert grams to moles, we need the molar mass of each element.
Molar mass of C = 12.01 g/mol
Molar mass of H = 1.01 g/mol
Moles of C = Mass of C / Molar mass of C
Moles of C = 85.64g / 12.01 g/mol = 7.13 mol
Moles of H = Mass of H / Molar mass of H
Moles of H = 14.36g / 1.01 g/mol = 14.2 mol
3. Find the empirical formula:
Divide the number of moles by the smallest number of moles to get the simplest ratio:
C: 7.13 mol / 7.13 mol = 1
H: 14.2 mol / 7.13 mol = 2
Therefore, the empirical formula is CH₂.
4. Determine the molecular formula:
To find the molecular formula, we need to know the molar mass of the compound.
Molar mass of empirical formula (CH₂) = (12.01 g/mol × 1) + (1.01 g/mol × 2) = 14.03 g/mol
Divide the molar mass of the compound (42.08 amu) by the molar mass of the empirical formula (14.03 g/mol):
Molecular formula multiplier = 42.08 amu / 14.03 g/mol = 2.999 (approximately 3)
Multiply the subscripts of the empirical formula by the molecular formula multiplier:
(C₁H₂)₃ = C₃H₆
Therefore, the molecular formula of the compound is C₃H₆.
To find the molecular formula of the compound, we need to first determine the empirical formula using the given percentages of carbon and hydrogen.
1. Convert the percentages to grams:
- Carbon: 85.64% of the total mass = 85.64 g
- Hydrogen: 14.36% of the total mass = 14.36 g
2. Convert the mass of each element to moles using their atomic masses:
- Carbon atomic mass = 12.01 g/mol
- Hydrogen atomic mass = 1.01 g/mol
Moles of carbon: 85.64 g carbon / 12.01 g/mol = 7.13 mol
Moles of hydrogen: 14.36 g hydrogen / 1.01 g/mol = 14.22 mol
3. Find the mole ratio by dividing each value by the smallest number of moles. In this case, the smallest number of moles is 7.13 mol (from carbon):
- Carbon: 7.13 mol / 7.13 mol = 1
- Hydrogen: 14.22 mol / 7.13 mol = 1.99 (approximated to 2)
4. Write the empirical formula using the mole ratio obtained:
The empirical formula is CH2.
Now, to find the molecular formula, we need to know the molar mass of the compound. Given that the formula mass is 42.08 amu, which is close to the empirical formula mass of 30.03 amu (1 carbon + 2 hydrogen), we can infer that the empirical formula is also the molecular formula.
Therefore, the compound's molecular formula is CH2.
Take a 100 g sample. This will give you
14.36 g H and 85.64 g C. Convert those to moles.
14.36/1 = ??
85.64/12 = ??
Now find the ratio of the two elements to each other. The easy way to do this is to divide the smaller number by itself (thereby assuring that you will have 1.00) for that number. Then divide the other number by the same small number. Round to whole numbers. That will give you the empirical formula.
To find the molecular formula, add the atomic masses to find the empirical formula mass. Then 42.08/empirical formula mass = ?? and round to a whole number. I will call that x, then the molecular formula is (CyHz)x.