[Ajayp]

I was wondering if it was possible for someone to explain me on more on how to approch these types of questions than to get the answer.  Thank You.

Quesion 1: 40.64 mL of 0.5000 M AgNO3 is added to 270.00 mL of water which contains 5.832 g K2CrO4. A red precipitate of Ag2CrO4 forms. What is the concentration, in mol/L, of unprecipitated CrO42-? Be sure to enter the correct number of significant figures. Do not include units in your answer. Assume Ag2CrO4 is completely insoluble.

Question 2:  The plastics industry uses large amounts of phthalic anhydride, C8H4O3, made by the controlled oxidation of napthalene: 2C10H8 + 9O2 ---> 2C8H4O3 + 4CO2 + 4H2O. Since some of the napthalene is oxidized to other products, only 70 percent of the maximum yield predicted by the above equation is actually obtained. How many kilograms of phthalic anhydride would be produced in practice by the oxidation of 100 kg of C10H8?

a. 76.1  
b. 81.0
c. 70.0
d. 30.0
e. 57.6  

Thank you to everyone who contributed in helping me out.  I am not in any rush to get the answer.

[Karakth]

Question 1

Hint: The very first thing you should do when confronted with questions like these is to write the chemical equation. From that you can get a ratio of how the chemicals react and can begin to work out what excess molecules you have.

Question 2

Hint: Same, look at the equation and calculate what yield you should get. And remember, work in moles not in mass.

[kha35a]

Hey Karakth, I was trying to work around this problem but I failed miserably I believe.  Is there anyway u can SHOW me how to derive the answer please. THanks

[Karakth]

I cannot give you the answer, obviously.

Ok, first write down the equation:

CrO₄^2- + 2Ag⁺ --> Ag₂CrO₄

Now, calculate how many moles of each reactant you have. I assume you know how to do this.

Obviously the number of moles of CrO₄^- is greater than the number of moles of Ag⁺, since there are leftover ions.

For the second question, calculate how many moles 100kg of C₁₀H₈ is. This will (theoretically) yield the equal amount of moles of phtallic anhydride. However, you only have 70% of that formed in practice. Then convert that number of moles (70% of the theoretical yield) to kg.

Hope that helped clarify things.

[kha35a]

It did very much.  Thanks a lot.  Just one more question: What should be the right number of significant digits for the first question?

[Karakth]

Assuming that your exam system uses the same method as mine to calculate significant digits, you will have to base it off the numbers you are given:

40.64, 0.5000, 270.00, 5.832

What are the amount of significant figures in each case?

I would say 4, but I am somewhat unsure of that answer.

[Borek]

40.64, 0.5000, 270.00, 5.832

What are the amount of significant figures in each case?

I would say 4, but I am somewhat unsure of that answer.

270.00 has 5. But it is the smallest number that counts.

[Karakth]

Possible fluke?

[kha35a]

thx a lot u guys...really appreciate the help