Today is 28 July 2009 -- Today we focused on the carbon dioxide evolution lab in our study of the Gas Laws

Students calculated how much vinegar (containing 5% acetic acid) did it take to react with a weighed amount of bicarbonate of soda.  The bicarb was put in a flask and the vinegar was put in a balloon.  The balloon was fastened to the flask and the vinegar tipped into it.  The carbon dioxide that was produced caused the balloon to inflate. The students determined the volume of the balloon.  From the volume, they determined how much bicarbonate had actually reacted and calculated a yield of actual to theoretical.  Formulas used were:

1.  Gross weight - tare weight = net weight

2.  moles = mass (in grams) / atomic weight (sodium bicarbonate)

3.  Stoichiometric ratio bicarb to acetic acid determined.

4.  grams acetic acid needed = moles x atomic weight of acetic acid.

5.  weight of acetic acid in grams x 100%/5% = grams of vinegar needed

6.  grams vinegar = ml vinegar

7.  balloon circumference (C) = Pi x diameter (d)

8.  d = 2 radius (r)

9.  Volume of balloon (V) = 4/3 x Pi x r(exponent 3)

10.  moles CO2 = P x V/R x T where R = gas constant (0.082 liters.atm/mol.K) and T is temperature in Kelvins

11.  K = oC + 273.15

12.  mole ratio CO2:NaHCO3

13.  moles NaHCO3 x atomic weight NaHCO3 = grams NaHCO3 actually reacted

14.  grams Actual / grams used in equation 1 x 100% = yield

Students were told to save the solution that results.  It should be clear.  If not, it means that some of the bicarb was left unreacted.  The solution, now sodium acetate, is something that we want to use in our study of solutions.  Specifically, when sodium acetate is in a "supersaturated solution," meaning there is more sodium acetate than should actually be capable of being dissolved, and a "seed crystal" is injected, the solution will crystalize and release heat.  It is packaged and used as hand warmers.  I would like to set you up to make hand warmers by evaporating the solution you have.  I need something to activate the solution, which is what I am now looking for.  Commercial warmers use a thin, stainless steel disk. 

Many of you need practice solving problems.  I'm giving you a few (below) to practice on:

1.  A "molar solution" is made by dissolving one mole of a substance in water, then topping up to 1 liter.  The concentration is thus one mole per liter.  What is the concentration of 4 grams of table salt, sodium chloride, in one liter of solution?

2.  Hydrochloric acid, HCl, is typically used at a concentration of 1 Molar, or one mole per liter of solution.  How many grams of HCl is needed to make a 1 Molar solution?

3.  Concentrated hydrochloric acid is 37.5% by weight.  It is made by bubbling HCl gas through water until the water is saturated.  How many grams of HCl gas are in 1000 grams of concentrated HCl acid?  How many moles is this?