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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?

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Thank You!!!

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• Lexi R
Lv 7

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

CH2

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

= 4

So molecular mass = empirical mass x 4

4 x (CH2)

C4H8

• Bobby
Lv 7

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 ,,