## ggrree 3 years ago I found this question in the chemistry section of a test. I need help with number 17-- I have no idea where to start. http://gyazo.com/8e8407570f9ec590c8485d47cf429beb (PS: the answer for 16 is B, confirmed with answer manual.)

1. eseidl

good question! lots going on here.

2. eseidl

Well, since you have the formula already from question 16, this is simple. The molar weight of CH is just (approx) 12+1=13 g/mol

3. eseidl

The hard question is #16...

4. ggrree

I only found the empirical formula for the gas, remember. The molecular mass depends on the MOLECULAR formula, which may or may not be CH. the gas could be CH, C2H2, C3H3, etc. so actually, I don't see how your solution is correct...

5. eseidl

oh...ic what you mean. I misread that part. I'll have another look.

6. dominusscholae

Ok....Ihave a solution. The mass in chamber 1 is all H20. THus, we can conclude that there was .9 grams of H20 originally. Converting this to moles gives approximately .05 moles of H20. We do the same to the material in Chamber 2 since that was all CO2, and we get .1 moles of CO2. By conservation of mass, we must conclude that there were .05*2 = .1 moles of H and .1 moles of C that were on the product side. Since the problem said the hydrocarbon only containted C and H, we can say that the original sample hydrocarbon contained .1 moles of C and .1 moles of H. From here, we can get the empirical formula by computing the ratio of C to H in the hydrocarbon. Since it is a 1:1 ratio, the empirical formula is CH!.

7. dominusscholae

for 17, we can compute the molar mass of the hydrocarbon by using this formula:\[Molar mass = density*R*T/P\]. Doing so for the hydrocarbon yields approximately 26 g/mol.

8. ggrree

Thanks, your solution for number 16 is similar to mine. The way you answer 17 is strange to me, though. Where do you derive that formula from?

9. dominusscholae

Ideal Gas Law: \[PV = nRT\]\[Density = mass/Volume.\] But in this case Mass = molar mass * n (number of moles). Thus n = Mass/molar mass. Solving for molar mass, we get\[Molar mass = Mass*R*T/(P*Volume)\]. But as established above, Mass = Mass/Volume. Using this arrives to the equation I gave.

10. ggrree

wow, that's really useful! I wonder why I never encountered that in class before... Thank you so much!

11. dominusscholae

Np :)