[Russian_916]

Hello All,

I am new to the forum, and am thrilled to have found this site.  I have been having trouble with Chemistry since High School, and I have come to terms with the fact that I am incapable of learning, or enjoying Chemistry.  It is just not for me.  I have a study guide for my final this Thursday, and I have answered all except these problems.  If someone could take the time to solve, or better yet explain the process, I would greatly appreciate it.

Stan

1.The concentration of a solution of HCl is 33.1% by mass, and its density was measured to be 1.147g/mL.  How many mL's of the HCl solution are required to obtain 10.0 g of HCl?  This seems quite simple, but I have no idea how to set it up....

2.  An experiment calls for 1.00 g of silver nitrate, but all that is avaialbe in the laboratory is a 0.50% solution of silver nitrate.  Assuming the density if the silver nitrate solution to be very nearly that of water because it is so dilute, determine how many mL if the solution should be used.  Once again, I dont know where to start...

3.  Calcium oxalate is very insoluble in water.  What mass of sodium oxalate is required to precipitate the calcium ion from 37.5 mL of 0.104 M CaCl2 solution?

4.  What mass of calcium carbonate should be taken to prepare 500.0 mL of 0.0200 M cacium ion solution?

[Wald_ron]

1)

you have how much mass you want (10g)
% mass (how much of the original mass is in your solution)(.331)
and density(mass/volume) (1.147g/mL)

find volume

The trick to these sort of problems is realizing that you're actually moving backwards through the problem, rather than forward , and also that everything is interconnected.

The other hard part about these problems can be the setup, the simplest way to do it is as follows:

(Lets pretend you were to try to convert grams to mols

g      mol
    x  ___  = mol      or we could even say that we are trying to find the weight           
                                3 grams of Mercury
         g


If we look on our periodic table it's easy to see that the molecular weight of mercury is 200 which is in grams per mol.


3 grams of mercury x    1 mol of mercury          = .015 mols of mercury
                                  ___________
                                200 grams of mercury


 The grams will cancel , if you remember from maths ,

3           4         
_ = 1     __ = 1     

3           4

In chemistry the same thing is also true with units, thus

g
_ = 1    and the units are able to cancel
g
           

Hope this helps