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What is the molecular formula of the compound?
A gaseous hydrogen and carbon containing compound is decomposed and found to contain 85.63% C and 14.37% H by mass. The mass of 258mL of the gas, measured at STP, was found to be 0.646g. What is the molecular formula of the compound?
- Lexi RLv 71 decade agoFavorite Answer
first work out the empirical formula by determining how many moles of each atom are in a sample of the compound.
Then use the volume at STP to work out moles of gas, and thus molar mass of the compound.
Then you can find the molecular formula from the molar mass and empirical formula
Start with 100 g, work out moles of each atom in the 100 g
mass H = 14.37 g
moles H = 14.37 g / 1.008 g/mol = 14.26 mol
mass C = 85.63 g
moles C = 85.63 g / 12.01 g/mol = 7.13 moles
Ratio of C : H
7.13 mol : 14.26 mol
Divide both numbers in the ratio by the smallest number
C : H
7.13 / 7.13 : 14.26 / 7.13
= 1 : 2
So empirical formula is
work out the moles of the gas form the volume at STP
STP is standard Temp and pressure, P = 1 atm and T = 0 deg C
At STP 1 mole of any ideal gas occupies a volume of 22.4 L
So if you know the volume at STP
moles = volume (L) / 22.4 L/mol
moles Compound = 0.258 L / 22.4 L/mol
= 0.01152 moles of compound
You now know that
moles = 0.01152 mol
and mass = 0.646 g
So you can find molar mass
moles = mass / molar mass
therefore molar mass = mass / moles
= 0.646 g / 0.01152 mol
= 56.1 g/mol
To work out the molecular formula divide the molar mass by the mass of the empirical formula
This tells you how many times to empirical formula fits into the molecular formula
empirical mass = 12.01 + (2 x 1.008) = 14.026 g/mol
molar mass / empirical mass
= 56.1 g/mol / 14.026 g/mol
So molecular mass = empirical mass x 4
4 x (CH2)
- BobbyLv 71 decade ago
It best to work out the molecular weight then the molecular formula first
1 mole occupies 22.4 liters ; in .285 liters we have .258 / 22.4 = 0.0115 moles
these weigh .646 g so 1 mole weighs .646 / .0115 = 56.08 g/mole
to calculate the composition of the compound we have
85.63% C or 86.63 % of 56.08 = 48g or 4 moles of carbon
14.37 % H or = 14.34 % of 56.08 = 8.04 g or moles of H
molecular formula is C4H8 ... simple ..
N B :you don't need to calculate the empirical formula first.
also the empirical mass is not a real chemical entity ??
in my solution we are only working with real chemical experimental data .. good luck ,,