Hello mdouglas04,
As requested, I have provided the procedure required to conduct the
experiment described in your question.
Experiment: Calculating the Mass of a Soluble Additive
Purpose: To determine the percentage of soluble additive contained
within a power which itself is inwater soluble.
Hypothesis: The powder product contains approximately 0.2% soluble additive.
Key Equipment:
Balance
Petrie Dish
Graduated Cylinder
Heat Lamp
Procedure:
1. Determine and record the mass (in grams) of the empty petrie dish
when it is clean and dry.
2. Repeat step (1) for the empty graduated cylinder.
3. Take a small amount of powder (half a teaspoon or less), place it
in the graduated cylinder.
4. Record the combined mass of the cylinder and powder, and subtract
the intial mass of the cylinder to obtain the intial mass of the
powder.
5. Slowly add small amounts of water (preferably filtered) to the
graduated cylinder while continuously swirling the cylinder to help
dissolve the powder entirely.
6. Repeat (5) until the powder has completely dissolved and no
particles are visible in the mixture.
7. Record the volume (at least 0.1 mL precision) of the solution
8. Carefully transfer some of the solution to the petrie dish. This
should certainly be less than a third of the amount required to fill
the dish. If multiple attempts are required, ensure the volume of the
cylinder is recorded each time (this ensures all 'lost' solution is
accounted for). Record the volume of the solution added to the petrie
dish.
9. Place the petrie dish under the heat lamp and leave it until no
liquid remains in the petrie dish. The final content on the dish
should resemble crystals (which can then be ground back into the
original powder if necessary but are the same mass)
10. Record the mass of the petrie dish with crystallized powder.
11. Subtract the original mass of the petrie dish to obtain the mass
of the crystals.
12. Calculate the percentage of the total solution used:
Total % of solution used = Volume of solution used / Total solution volume
13. Use the percentage from (12) to calculate the amount of powder
actually used for crystallization:
Mass of powder used = (Total powder mass)* (Total % of solution used)
This calculation is justified by the fact that a solution contains a
uniform concentration of solute by volume.
14. Compare the mass from (13) to the mass found in (11).
Results:
If the two calculated masses are equal, the the powder does not
contain any additive. However, since our hypothesis states that we
expect there to be a discrepancy, we can calculate the amount of
soluble additive by simply calculating the % difference between the
two masses:
% additive = (Powder mass  Crystal mass)/(Crystal mass)
I hope this provides you with a clear understanding of how this type
of experiment should be conducted. If any of the information above is
unclear, please let me know with a clarification request.
Cheers!
answerguruga 
Request for Answer Clarification by
mdouglas04ga
on
21 Dec 2004 13:30 PST
Hello answerguru,
Thank you for answering my question.
I'm not clear on step 14, where you compare the powder mass from step
13 with crystal mass from step 11. They could never be equal, right?
In any case, here is my understand of your proposal:
Let's say I use 100 grams of powder and mixed with distilled water to
create a solution, and let's say the volume of the solution (water +
powder) is 1 liter.
Then I transfer 500 ml (1/2 liter) of solution to the petrie dish, use
heat lamp to evaporate the water, and in the end I get 0.1 gram of
crystals, this is the result of step 11.
Now since I used 50% of solution, based on the step 12 and 13, the
amount of powder actually used for crystallization is: 100 grams x 50%
= 50 grams.
So now I have 50 grams of original powder versus 0.1 gram of crystals,
the % of additive should be (50  0.1)/50
Please confirm this is the correct understanding.
Lastly, in regards to step 7 where you record the volume of the
solution, should you be recording the volume of the water you added?
The volume of step 7 is really the volume of water added PLUS the
powder.
Thanks again.

Clarification of Answer by
answerguruga
on
21 Dec 2004 23:16 PST
Hi again,
In step 14, the comparison of the two calculated masses can only be
equal when there is no additive. In your case, since you know an
additive exists, there will certainly be a difference.
Your outlined understanding is correct  you have got the hang of it now :)
Your clarification regarding step 7 is correct, but difficult to
obtain unless you know the level of solubility. Recall that you are
adding small amounts of water until no solid material remains. You
could do a test run to find out approximately how much water is
required for a given amount of powder and then repeat by using a
precise amount of water (recording the volume) and then adding a
predetermined mass of powder.
Thanks for using Google Answers!
Cheers,
answerguruga
