What Is the Molecular Formula of the Alkane in This Chemistry Problem?

[daphap]

Eurgh, for some reason I can't get my head around this chemistry homework!

"When 20cm^3 of this alkane were burnt in 200 cm^3 of oxygen (an excess), the residual gases occupied 150cm^3. On shaking with an aqueous potassium hydroxide, this volume decreased to 70 cm^3. (All at rtp and pressure). What is the molecular formula of the alkane."

Oh yeah, and I devised that the empirical formula of th alkane was C2H5.

I am aware I have to show my work to get help... but I really don't know where to start. If someone good guide me through a similar problem with alter numbers I would greaty appreciated. Thanks.

 

[SN1987a]

How did you devise the empirical formula of the alkane was $C_2 H_5$?

If you're sure this is the right empirical formula, then you're basically done.

You now have to know a bit about alkanes.

An alkane is a chain of single-bonded carbons with enough hydrogens attached to each carbon. Knowing that each carbon needs fours bonds to be "happy", you can find a relationship between the number of carbons in an alkane and the number of hydrogens.

You now use the empirical formula, which also gives you another relationship between the number of carbons and hydrogens present in your alkane. With those two formulas in hand, you can figure out precisely how many carbons and hydrogens a molecule of your alkane is made up of.

 

[quicknote]

Hello there,

First of all, don't get discouraged by your difficulty with this chemistry homework. It's completely normal to struggle with certain concepts and problems in science. The key is to keep practicing and seeking help when needed. Let's break down this problem step by step to help you understand it better.

1. The first thing we need to do is to understand the given information. We have an alkane (which is a type of hydrocarbon) that was burned in the presence of oxygen. The residual gases, after the reaction, occupied a volume of 150cm^3. This means that the alkane reacted completely with the oxygen and used up all of it.

2. Next, we see that the remaining gases (residual gases) were shaken with an aqueous potassium hydroxide. This resulted in a decrease in volume from 150cm^3 to 70cm^3. This decrease in volume is due to the formation of a new compound, which is potassium carbonate in this case. This tells us that the alkane had a functional group that reacted with the potassium hydroxide to form potassium carbonate.

3. Now, we need to use the given information to determine the molecular formula of the alkane. We know that the empirical formula is C2H5, which means that for every 2 carbon atoms, there are 5 hydrogen atoms. This gives us a ratio of 2:5. We also know that the alkane reacted with oxygen in a 1:10 ratio (20cm^3 of alkane reacted with 200cm^3 of oxygen). This means that the number of carbon atoms must be twice the number of oxygen atoms.

4. Using this information, we can set up an equation to solve for the number of carbon and oxygen atoms. Let's use x for the number of carbon atoms and y for the number of oxygen atoms.

x = 2y (since the number of carbon atoms is twice the number of oxygen atoms)

x + y = 10 (since the alkane and oxygen reacted in a 1:10 ratio)

Solving these equations, we get x = 4 and y = 2. This means that the molecular formula of the alkane is C4H10.

I hope this helps guide you through the problem and understand it better. Remember, practice makes perfect and don't hesitate to seek help when needed. Good luck with your chemistry homework!