Finding the Molecular Formula
posted by Amy on .
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.
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
You used the wrong atomic weight for C. It is 12.011, not 14.011
I'm still unsure as to how to find the molecular formula. I found the number of moles of CH.