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chemistry

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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.

  • chemistry -

    Did you read my response to your earlier post of this question? If not, please do so. Some of your answers don't make sense no matter what procedure you used. Others I don't know about because I don't know what you did. (For example, I know 4 and 7 are not correct.)

  • chemistry -

    I did post the procedure on the other post:)

  • chemistry -

    I think my older post was lost within the forum so I just reposted, sorry.

  • chemistry -

    Thank you. That's a lot of typing but now I know what you did. I have copied the procedure to this post to make things a little easier if we need to refer to it again.
    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 =/

    Here are your answers with my response to each in bold.

    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?

    exactly right.



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

    No. You added NaCl in excess to ppt all of the silver. Washing the ppt with water (usually a little HNO3 is added to the wash water) removes the NaCl remaining in the solvent during the filtration step

    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.

    You're right, the answer is no, but the reasoning is wrong. Remember you dissolved the dime in HNO3 and pptd the AgCl in the same solution (or at least in a diluted solution) so AgCl is not soluble in HNO3.

    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.

    I think the calculated percent will be too low. The air in the crucible will be hot, hot air has a lower density than cold air, the mass will be lower for the crucible with the AgCl. If you weighed the empty crucible first (and correctly), the difference in mass will be too small and the measured mass of the AgCl will be too small. Check my thinking.Technically, I would answer that you could have either too high or too low BECAUSE one NEVER places a hot crucible on an analytical balance BECAUSE it disturbs the air currents which prevents an accurate weighing in the first place.One could argue that the hot crucible would heat the air in the balance chamber, this would change the buoyancy of the air thus changing the weight but I won't get into all of that.

    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.

    Because silver and copper (the constituents of the dime) will not dissolve in HCl. Remember both Ag and Cu are below H in the activity series.

    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.
    I don't know what kind of special filter paper you used; however, I assume it was paper that was prepared to weigh as little as possible so that the mass of the AgCl + paper was essentially the same as the mass of the ppt alone. However, that is a guess on my part so don't take this particular answer to the bank.
    I hope this helps. Thanks for your answers. It helps to know your thinking; it makes the question easier to answer.

  • chemistry -

    Wow, thank you so much! There is one more question,sorry><.

    2. Why is a twofold excess of chloride added to precipitate the silver?
    The ionic product has to be more than constant in order for the particular compound to precipitate. In this lab, the exact concentration Ag+ and Cl- are not determined. The extra amount of Chloride helped make sure that there is enough Chloride to make sure that the precipitate will follow through.

  • chemistry -

    Your answer is essentially right but a little wordy and in spots it sounds like you aren't sure of the answer so let me rephrase it.
    To ppt the AgCl, one must exceed Ksp for AgCl. To make sure that ALL of the AgCl ppts (remember AgCl does have a slight solubility in water--I think I remember it's about 1.43 mg/L) one adds an excess of NaCl. The excess Cl^- is a common ion to AgCl and the AgCl(s)--> Ag^+(aq) + Cl^-(aq) reaction is shifted to the left by Le Chatelier's Principle resulting in lower solubility loss of AgCl solid. In other words, the solubility of AgCl is decreased in excess NaCl. By the way, check with some of your class mates on the hot crucible question/answer. I may not have interpreted everything exactly the same as it should be interpreted. One other point I didn't make. Generally, in analytical labs where AgCl is determined, one does not use filter paper for the exact reason mentioned in the question. The paper weighs too much to ignore for REALLY exacting work. So we use filter crucibles (crucibles made with a sintered porcelain bottom. They come in different porosities for use with ppts that have different sizes,) The difference between the empty xble and the xble containing the ppt is the mass of the AgCl alone and there is no paper to mess up the works.

  • chemistry -

    Thank You so much DrBOB222....You really helped me

  • chemistry -

    This was a lot of help I have to do the lab write up and I was confused on the second question and was able to double check my other anwsers thanks so much!

  • AP Chem -

    Thanks Cinco for helping my dad cheat!

  • chemistry -

    Thanks for the help!

  • AP Chem -

    This post helped immensely, thank you so much and merry Christmas!

  • chemistry -

    for number 8 the crucible weighs more than a piece of filter paper..
    it's because you have to heat the crucible, and the paper would catch on fire (danger) :D

  • chemistry -

    why do you have to write on filter paper with pencil?

  • chemistry -

    You don't write in filter paper.

  • chemistry -

    You write on the filter paper so that you know it is your paper.

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