- Find the mass of a clean and dry 125-mL (or similarly sized) Erlenmeyer flask (check page 7
if you are unfamiliar with what this flask looks like) using an electronic balance. Enter the weight on the data sheet as mass 1. If you plan to mark the flask with any identifying mark or tape, be sure to do this before any mass is determined and recorded.
- Add 10 scoops of the assigned unknown mixture into the pre-weighed Erlenmeyer flask. Record the weight of the flask + unknown mixture on the same electronic balance used previously; record as mass 2. Make sure to record the unknown number as well. By the difference between mass 2 and mass 1, one can calculate the sample mass of the starting unknown mixture being used.
- Obtain approximately 30 mL of dilute hydrochloric acid (HCl) into a small beaker from the main storage bottle and take this back to your work area. Take care in handling this acid as it can cause burns of clothes and exposed skin if spilled.
Dilute hydrochloric acid (HCl) can harm eyes, skin, and clothing. Handle with care. Any acid spilled on the skin or splashed into your eyes must be rinsed with a large volume of water. Any acid spilled on benchtop or cart, should be first neutralized with sodium bicarbonate, NaHCO3, then cleaned up with paper towels.
- Slowly pour about 5 mL of the acid into the Erlenmeyer flask with the unknown sample and stir slowly with a glass stirring rod; do not remove the stirring rod from the Erlenmeyer flask as this may cause loss of product if any mixture or sand has adhered to the rod. You should see a large amount of bubbles being produced as gas (CO2) escapes the mixture upon adding the acid. When the bubbling stops, add another small portion of the acid while stirring again.
- Repeat the addition of small amounts of acid until there is no more bubbling. Stir the reaction mixture to ensure complete reaction of the sodium carbonate (Na2CO3) before you add any more acid. The key here is to use the MINIMAL amount of acid needed to react all of the Na2CO3 in the unknown mixture as a matter of personal safety.
- When you feel that the addition of any more acid will not produce any more CO2 gas, call for the instructor to verify. Do not proceed further until your reaction mixture has been checked by the lab instructor.
- Once given instructor approval to continue, you will be transferring the reaction mixture from the flask to a drying beaker to make washing and drying of the sand more efficient. Find the mass of a clean and dry 250-mL beaker. Record this data as Mass 3 on the data sheet.
- Swirling the contents of the Erlenmeyer flask into a homogeneous mixture and quickly pour the mixture into the pre-weighed drying beaker. Make sure to get as much of the sand product out of the flask and into the drying beaker.
- Let the drying beaker sit undisturbed for 2 minutes to allow for the sand to settle to the bottom of the container. Carefully decant the upper liquid portion of the mixture into the sink. Take care to prevent any loss of sand during the decanting process. See “How to decant” on next page for guidelines.
- Add about 20 mL of distilled water to the Erlenmeyer flask and swirl to catch any sand that might have been stuck inside the flask and then add this water wash to the drying beaker. This water wash is to dilute any residual acid that may have been remaining with the sand. Stir with a stirring rod for about 15 seconds; do not remove the stirring rod from the beaker.
Allow the beaker to sit undisturbed for 2 minutes to allow for the sand to settle and then carefully decant the liquid into the sink. Make absolute sure that no sand is lost during the decanting process.
- Wash the sand in the beaker with another 20 mL portion of distilled water. The second wash is to ensure complete removal NaCl from the sand. Stir the mixture for 15 seconds then allow the sand to settle as before. Then carefully decant the liquid into the sink making sure that no sand is lost in the process.
- To remove the water from the sand and speed up the drying process, wash the sand with five (5) mL of methanol. Make sure to keep the methanol away from the drying oven and any open flames as methanol is highly flammable. Stir and then allow settling before decanting the liquid level off. Place the decanted methanol into the “waste methanol” jar.
Methanol is flammable. Keep away from open flames or heat sources.
- Using beaker tongs (or similar) place the drying beaker with the “wet” sand in the dying oven. Be cautious in placing your dish in the hot oven especially if there are several beakers already in place. Record the time the beaker is placed in the drying oven.
- After a period of time in the drying oven, remove the beaker (don’t forget to use the beaker tongs!) and allow it to cool on the bench top. Record the time the beaker is removed from the drying oven. Determine the minutes the beaker was in the drying oven. The beaker should be cool enough so that you feel NO heat from the outer surface when placed near the back of your hand. Placing a beaker too hot to touch on the scale can melt the weighing pan and destroy the scale. The basic rule of thumb is to cool the beaker 1⁄2 the heating time as a minimum.
- Weigh and record the mass of the drying beaker + sand using the same balance you previously used.
- Replace the drying beaker back into the drying oven for another period of time. After which you will cool and record the mass of the evaporating dish + sand again. If this 2nd mass is within ± 0.05 grams of each first, then you have completely dried the sand and have reached constant mass. In this state the sand is assumed to be completely dried and the mass will remain constant no matter how much more heating time is applied. Record the constant mass value as mass 4 on the data sheet.
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