# Homework Help: Physical Science Calculations NEED HELP

1. Oct 10, 2005

### ckbppb

I need help, please! I am totally and completely lost with this problem. Unfortunately, my book does not give any examples for me to go by.

Here is the problem...

In the following reaction, 24 g of CH4 (methane) react completely with 96 g of O2 to form 66 g of CO2. How many grams of H20 are formed?

CH4 + CO2---->CO2 + 2H2O

Where do I begin on this problem? TIA for any help provided.

2. Oct 10, 2005

### Mozart

Does the methane react with CO2 or O2?

(Sorry, I edited instead of quoted ~ Moonbear )

Last edited by a moderator: Oct 10, 2005
3. Oct 10, 2005

### Moonbear

Staff Emeritus
1) You'll need to start out finding the molecular weights of each molecule in your reaction.
2) Then use that information to find the number of moles of starting and ending product.
Hopefully, you'll be able to see from there how you can use your given information to solve for your unknown. If not, come back with your answers to 1 and 2 and we'll help you get to step 3.

4. Oct 10, 2005

### Moonbear

Staff Emeritus
It has to be O2.

5. Oct 10, 2005

### Mozart

Would you have to balance the equation first Moonbear? I ask because as it is written there are more oxygen atoms on the products side.

6. Oct 11, 2005

Yeah, you would have to balance out the atoms. The mole ratios need to be correct for the stoichiometry.

7. Oct 11, 2005

### ckbppb

WOW! Thanks for all the friendly postings. I really appreciate it. However, this is simple Physical Science 8th grade style. We haven't gotten into the complicated stuff quite yet. At any rate, I ended up figuring it out for myself. To my surprise, my science teacher said I did everything correctly. Here is what I came up with, just in case there are other posters working on the same type of problem.

Here are the knowns:
mass of CH4= 24g
mass of O2= 96g
mass of CO2= 66g
mass of reactants=mass of products

CH4 + 2O2---->CO2 + 2H2O
24 g + 96 g ---->66 g + ?
120 g – 66 g = 54 g
24 g + 96 g ---->66 g + 54 g
H20= 54 g

Also, this follows the law of conservation of mass, meaning the mass of all substances that are present before a chemical change equal the mass of all the substances that remain after the change.

8. Oct 12, 2005

### Moonbear

Staff Emeritus