chemistry; Lab questions help=)

was hoping someone can help me check if these answers make sense for a silver copper alloy lab.
3. Why don’t you have to weigh the sodium chloride on a sensitive balance?
This one I am not sure about, It is because an excess of NaCl is needed to make sure the solution precipitates?

4. Why is it necessary to wash the precipitate?
To receive an accurate reading on the mass of the precipitate.

5. Will the nitric acid in the wash water interfere with the weight of the silver chloride? (Nitric acid is prepared by dissolving the gas N2O5 in water.)
No because it is just excess water.

6. If the crucible containing the silver chloride is not cool when its mass is determined, will the calculated percent be too high or too low? Why?
It would be too high because the metal scale will contract with the heat and it will throw off the measurement.

7. Why don’t we just use hydrochloric acid to both dissolve and precipitate the silver?
Because Hydrogen is soluble and no precipitate willl form.

8. Why is a special filter crucible rather than plain filter paper used?

The special filter is much lighter and it will give a more accurate reading on the final mass of the precipitate.

asked by Dina
  1. Perhaps another tutor will know the details of the lab but I don't. If you care to post a summary of the procedure perhaps I can help.

    posted by DrBob222
  2. 1. Find the mass of the alloy.

    Obtain a sample of dime and determine its mass on an analytical balance.
    2. Dissolve the silver.

    Put the dime piece in a clean, 100-mL beaker and bring it over to the fume hood.
    Within the fume hood, carefully pour 10 mL of 6 M nitric acid over the dime sample.
    What is the limiting reagent in the reaction between the dime and the acid?
    Which one do you have to measure precisely and which reagent is in excess?
    Don’t waste your time being precise with all measurements if you don’t have to be.
    Cover the beaker with a watch glass so none of the solution spatters out.
    It may be necessary to gently heat the solution, so that all the alloy dissolves.
    3. Precipitate the silver.

    Assume the dime sample is 100% silver and use the mass of the sample to determine the amount of sodium chloride that would be necessary to precipitate all the silver.
    Use a balance to weigh out two times this amount of sodium chloride.
    Does this need to be measured precisely (i.e. do you need to use an analytical balance for this)?
    Dissolve the sodium chloride in 25 mL deionized water.
    Does the volume need to be measured precisely?
    Remove the watch glass from the first beaker, and slowly add the sodium chloride to the dissolved silver. (Be careful of splashing, because of the acid present.)
    Stir with a stirring rod and use your wash bottle to rinse any solution clinging to the rod into the beaker.
    Gently heat (without boiling) the solution for about 15 minutes. This will cause the precipitate particles to grow larger, so they are easier to filter.
    Alternatively, you can cover the beaker with Parafilm® and allow it to stand overnight. This, too, will allow the precipitate particles to grow larger.
    4. Filter the silver chloride.

    Insert the stopper on the stem of Büchner funnel into the mouth of the "armed" Erlenmeyer flask.
    Connect one end of the hose to the flask arm and one end to the aspirator on the faucet.
    You may want to set up a ring stand and single buret clamp to hold the flask upright.
    In the 250 mL beaker, place about 150 mL deionized water and 2-3 drops of 6 M HNO3.
    This will serve as the rinse solution.
    The addition of acid to the rinse will prevent the precipitate from "peptizing," or forming extremely small particles that will run through the filter.
    Obtain a piece of filter paper and write your initials on it with pencil.
    Try to handle the filter paper as little as possible, so you do not deposit dirt and body oil on the paper.
    Using an analytical balance, find the mass of the filter paper.
    Place the filter paper at the bottom of the funnel and turn on the faucet/aspirator.
    Wet the filter paper with the rinse solution to make sure the vacuum is working.
    Carefully pour the silver precipitate and solution into the funnel.
    Use your stirring rod to direct the pouring.
    Rinse the sample beaker with the rinse solution several times, each time pouring the rinse into the funnel.
    You may want to use the rubber policeman to "squeegee" any of the remaining precipitate out of the beaker.
    5. Dry the silver chloride and determine its mass.

    Using an analytical balance, find the mass of a clean evaporating dish.
    Use a microspatula to carefully remove the filter paper and precipitate from the Büchner funnel.
    Place the filter paper in the evaporating dish and then place the dish in the drying oven.
    Let the sample remain in the oven overnight and then find the mass of the filter paper and evaporating dish together.
    6. Calculate the percent silver in the dime.

    Use your knowledge of stoichiometry and percent composition to calculate the percent silver in the alloy.

    this was the basic procedure, I hand written mine so =/

    posted by Dina

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