SRL Archive - Science

My honors chem teacher gave us a sheet with some challenge problems we need to do. here are a few i need help with (they are from the AP Text book, supposedly).

1. A compound has an empirical formula of C3H5"X"3. a 7.000g sample was determined to be 0.04751 mol of the compound. What element is "X"? (I got Chlorine, I'd appreciate it if someone checked it out though)

2. An unkown compound contains only carbon and hydrogen (a hydrocarbon). Complete combustion of a sample of this compound produced 2.641 g of carbon dioxide and 1.442 g of water as the only products. Find the empirical formula of the compound. Its molar mass is 44 g/mol. What is its molecular formula? (For the emp formula i got H2C, and for the molecular formula i got H6C3)

3. You have a mixture that contains sodium chloride (NaCl) and sodium chlorate (NaClO3). You want to determine what % of the mixture is sodium chlorate. You heat a 8.762 g sample of the mixture intensely in a crucible causing the sodium chlorate to decompose into sodium chloride and oxygen gas. The oxygen escapes from the crucible. The sodium chloride in the original mixture does not react. The mass of the solid in the crucible after the reaction is 6.503 g (this is the original NaCl and the NaCl produced from the reaction.) What was the % of sodium chlorate in the original mixture?

4. An unknown compound contains C, H, and O. Complete combustion of a 10.68 g sample of this compound produced 16.01 g of carbon dioxide and 4.37 g of water as the only products. Find the empirical formula of the compound. Its molar mass is 176.1 g/mol. What is its molecular formula?





Thanks guys :norris:

I don't even remember empirical formulas.

So I give you this video instead. http://www.khanacademy.org/video/molecular-and-empirical-formulas?playlist=Chemistry

Well, I can't be bothered with no. 4 (sorry! :)) because it would require me looking up stuff/drawing diagrams since I can't remember exactly how hydrocarbons react but no. 3 is fairly straightforward:

You have 8.762g and you end up with 6.503g, thus there were 2.259g of oxygen atoms.
Molar mass of oxygen = 16g, so 0.1412 mol of oxygen.
Each NaClO3 has 3 oxygen so there was 0.04706 mol of NaClO3.
Molar mass of NaClO3 = 23 + 35.5 + 16*3 = 106.5g
So there was 106.5*0.04706 = 5.012g of NaClO3 in the original mixture.
This is 57.20%.

Disclaimer: I only did chemistry for one year in high school :) We got problems like this though.

From: any_one
Well, I can't be bothered with no. 4 because it would require me looking up stuff/drawing diagrams since I can't remember exactly how hydrocarbons react but no. 3 is fairly straightforward:

You have 8.762g and you end up with 6.503g, thus there were 2.259g of oxygen atoms.
Molar mass of oxygen = 16g, so 0.1412 mol of oxygen.
Each NaClO3 has 3 oxygen so there was 0.04706 mol of NaClO3.
Molar mass of NaClO3 = 23 + 35.5 + 16*3 = 106.5g
So there was 106.5*0.04706 = 5.012g of NaClO3 in the original mixture.
This is 57.20%.

Disclaimer: I only did chemistry for one year in high school :) We got problems like this though.

Thanks, that's what i got.

And as for the thing about hydrocarbons, you just have to know that they react with O2 in the air to form CO2 and H2O haha

Yeah.. still it's a bit more involved. Good luck :p

4. An unknown compound contains C, H, and O. Complete combustion of a 10.68 g sample of this compound produced 16.01 g of carbon dioxide and 4.37 g of water as the only products. Find the empirical formula of the compound. Its molar mass is 176.1 g/mol. What is its molecular formula?

Ignoring sig figs:

CxHxOx

4.37g of water are produced, which means 4.37 / 18.02 IIRC = 0.209767 moles of H2O
Therefore there are (2 moles H per 1 H2O) = 0.4195 moles of Hydrogen, and 0.209767 moles of O from water

16.01 g of carbon dioxide = 16.01 / ~44g/mol CO2 = 0.3639 mol
Based on how I did the ratios above, 0.3639 mol C and 0.7277 mol O

mol C = 0.3639
mol H = 0.4195
mol O = 0.7277 + 0.2097

Then what do you do is find the ratio there, when you have the total moles, divide the lowest number into all of them. If you get something weird like 0.5, multiply everything by 2. If you get a REALLY weird number, chances are you're doing it wrong since at this level they don't give you insane questions that have 0.810429 in them (though that could work).

Then with your final compound, you will have figured out
CxHyOz (where x/y/z are numbers)

Hopefully I did this right… :P Haven't done this for 3 years.

Source: I'm an almost graduated university science student