# Finding the Molecular Formula

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A certain compound containing only carbon and hydrogen was found to have a vapor density of 2.550 g/L at 100 degrees C and 760 mm Hg. If the empirical formula of this compound is CH, what is the molecular formula of this compound?

I had some ideas as to how to solve this problem.

First, I thought of using the Specific Heat Formula to find a value, that could resemble a known element.

Q = m CP delta T
1 = m(2.550)(373.15)
1 = m(951.53)
m = 0.00105 g

At this point, I'm really stuck. I know that to find the molecular formula of a compound, I need to divide the empirical mass over the molecular mass. However, I am not sure how to find both masses. I was thinking to find the empirical mass I could calculate CH.

C = 14.011 g
H = 1.007 g

CH = 14.011 + 1.007 = 15.018 g

This appears to be the molecular mass, but I still need the molecular mass.

• Finding the Molecular Formula -

P V = n R T

You know P, V is one liter, you know T and R
so solve for n

n is the number of moles of the gas (it will be a small fraction of a mole) in 2.55 grams

or the compound is 2.55/n grams per mole

CH you said is 15 grams per mole

If it were C2H2 it would be 30 grams per mole etc

• Finding the Molecular Formula -

You used the wrong atomic weight for C. It is 12.011, not 14.011

• Finding the Molecular Formula -

I'm still unsure as to how to find the molecular formula. I found the number of moles of CH.

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