Determination of the Chemical Formula Put on lab coat and safety - - PowerPoint PPT Presentation

Determination of the Chemical Formula

• Put on lab coat and safety goggles
• Turn off your mobile phone in the lab
• Put your school bag in the drawer or cabinet, do not put on aisle
• Put pre-lab report on lab bench for TA to check and sign
• Hand in “Lab Safety Certification and Identification” with photo,

signature and contact information on it

Collect laboratory apparatus:

• One large test tube and a drying tube (clean and oven dry)
• 250 mL Erlenmeyer flask, thistle tube and rubber tube
• Alcohol lamp, matches, and wind shields
• Ring stand with two extension clamps

Prepare personal apparatus:

• One clean 100 mL beaker (take 20 mL of 6 M HCl)
• 10 test tubes (collecting hydrogen gas over water)

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(2020/05/07 revised) Video on YouTube (Click)

Apparatus and Set up

2 Thistle tube

Hydrogen gas generator Copper oxide Extension clamp Collecting gas

• ver water

Drying tube Zinc granules Cotton

Ring stand Plastic basin

Clamp Collecting gas over water Reduction-Oxidation

Do not take the extension clamp off the stand on your lab bench

Objective and Principle

▪ Objective: Determine the empirical formula of copper oxide by the analytical method ▪ Techniques: －Weighing chemicals －Preparing and collecting hydrogen gas over water －Using an alcohol lamp

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Cu : O =

𝑵𝒃𝒕𝒕 𝒑𝒈 𝑫𝒗 𝟕𝟒.𝟔𝟓𝟕

:

𝑵𝒃𝒕𝒕 𝒑𝒈 𝑷 𝟐𝟔.𝟘𝟘𝟘𝟓

Cu : O (Mole ratio)

◼ Chemical reactions:

Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g) H2(g) + CuO(s) Cu(s) + H2O(l) ⎯→ ⎯

Reactant, black Product, red

Step 1: Prepare the Drying Tube

▪ Stuff both ends of the drying tube with small piece of cotton to keep CaCl2(s) from falling ▪ Prepare the drying tube on the plastic plate to prevent CaCl2 scatterring all over the bench ▪ Cotton and CaCl2(s) must not be stuffed too tightly to

• bstruct hydrogen gas flow

▪ Put the cap of CaCl2 back on the bottle after use

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Step 2: Filling Large Test Tube with Copper Oxide

▪ Wash clean, oven dry, and cool the large test tube ▪ Weigh and record the accurate weight of the tube (W1) ▪ Use the small end of the spatula to put 1 g of copper oxide in test tube ▪ Copper oxide should be placed in the center of the test tube ▪ Accurately weigh again (W2) Note 1: use the same analytical balance throughout the experiment Note 2: Avoid the test tube touching the wind shield that causes error in weighing

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Step 3: Set up the Apparatus

▪ Add 15 g of zinc granules into Erlenmeyer flask ▪ Use the clamp to hold the test tube and Erlenmeyer flask ▪ Avoid clamping the test tube where the copper oxide is placed ▪ The thistle tube should be as close to the bottom of the flask as possible (Use a rag to cover the thistle tube and adjust its height by rotating slowly to avoid the breakage and get injured)

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Step 4: Preparation of Collecting Gases over Water

▪ Fill the plastic basin with 2/3 water ▪ Place 10 test tubes into water and filled with water ▪ Hold the end of the test tube, keep the opening under water, and avoid the air getting into tube

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Check Set up by TA

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• Proper filling of drying tube
• Firmly connection of apparatus
• Suitable position of two clamps
• Airtight fitted of 3 rubber stoppers

Notice

• HCl(aq) is strong acid.
• The hydrogen gas generated is explosive.
• Do not close the windows or safety doors.

Step 5: Generate Hydrogen Gas

▪ Use 100 mL beaker to take 20 mL

• f 6 M HCl

▪ Pour the HCl(aq) through the thistle tube all at once ▪ Immerse the end of the thistle tube underneath the liquid ▪ Collect gases over water at once

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Step 6: Collect Gases over Water

Collecting Gases Over Water: ▪ Reverse the test tubes in water ▪ Put rubber tube into the opening of test tube to collect gas over water ▪ When test tube is filled with gas, stand test tube upside down on table (opening pointed downward) ▪ After collecting 10 test tubes, use matches to bring flame to the opening of the tube and test for “pop” sound ▪ Loud pop sound in the beginning (Mixture of air and hydrogen gas) ▪ Pop sound should become less and less noticeable when system is filled with H2 * When the pop sound becomes unnoticeable, it means the air in the system has been pushed out by the hydrogen gas. * Notice: start heating only after this step is completed. 10

Notice of using alcohol lamp

• Check the length of the wick of alcohol lamp
• Put out the fire first and use a funnel when adding alcohol
• The alcohol should be half filled

Step 7: Heat and Start the Reaction

▪ Add another 20 mL of 6 M of HCl(aq) to ensure generating of hydrogen gas continuously * Do not take too much HCl(aq) to reduce wasting of chemicals ▪ Light the alcohol lamp to begin heating

Take rubber tube out of water to prevent the water flow into system

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Step 8: Heating Evenly

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▪ Heat the reacting test tube from right to left evenly ▪ Heat evenly to eliminate the water condensing inside the tube ▪ Do not burn the rubber stopper and the clamp ▪ Use the wind shield to shield the wind ▪ Heat until there is no more visible change in its content ▪ Stop heating, but keep hydrogen gas flowing, and let the system cool down

Step 8: Weighing after System Cools to R.T.

▪ Maintain hydrogen gas flowing during cooling the system ▪ Disconnect the test tube after cooling to room temp. ▪ Accurately weigh the test tube and product copper with the same balance (W3) ▪ Calculate the mass of Cu and O, respectively ▪ Recycle the cotton, CaCl2, zinc granules (rinse with water), the copper produced into designated recycling container ▪ Recycle 6 M HCl into heavy metal recycling bin

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Notice

▪ Accomplish data analysis and hand in lab report to TA at the class. ▪ Lab report shall include calculation in detail as well as experimental results with suitable significant figures and units, as 1.10 g, 359.12 g, and 359.12 mL. ▪ Bring scientific calculators to every experiment ▪ Shall you have any questions, feel free to discuss with TA, always be prepared before class ▪ Actively offer your questions when hand in lab report to TA ▪ After the lab: －Clean and return the large test tube and drying tube. －Clean up the lab bench and hot plate －Tucked the chairs underneath table －Count and replace equipment in the cabinet underneath the bench according to equipment list

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Condensed report

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POST-LAB Assignments

◼

When you accomplish the experiment: 1) FIND TA to check your recording and get signature (Hand in lab certification and lab report) 2) Find ASSISTANT TA to check your clean up and apparatus, then get signatures 3) GROUPS ON DUTY shall help TA to recover the lab environments ▪ Take off lab coat and goggle only when you STEP OUT LAB ▪ Lab report (condensed report): ▪ A subgroup (person in charge): summit lab report including prelab report, lab records, and data analysis results ▪ B subgroup: summit your prelab report

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T1 - Alcohol Lamp

▪ Inspect the lamp before each use to make sure there are no cracks, chips or defects in the glass body. ▪ Adjust the wick height to about 3 mm from the top of the metal cap. ▪ Fill the lamp with denatured or 95% ethanol through a suitable funnel to about ½ to 2/3 volume. ▪ Light the alcohol lamp using a match. ▪ Use the lamp only in an upright position. ▪ Keep all combustible materials (clothing, paper, books, chemicals, etc.) away from the lamp when in use. ▪ Never use book or other items to raise the alcohol lamp. T1 Video on YouTube (Click) ▪ Use windshield to block wind and do not use books. Do not tilt lamp to avoid alcohol leakage while heating. ▪ Adjust device upon heating to an appropriate height. ▪ After use, cap should be put on to insulate air and extinguish the flame instead of blow it out. If lamp will not be used for a long time, alcohol inside should be returned to storage bottle. ▪ If lamp is overturned and cause small fire, cover the fire quickly with a wet rag and do not remove the rag immediately while extinguished, or flame may ignite again. Use fire extinguisher instead if a big fire happens. Inform teacher as soon as possible in such emergency and after fire is extinguished, immediately open doors, windows and exhausts to expel alcohol vapor in lab.

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T4 - Collection of Gases

• ver Water

◼

Collection of gases over water is used to collect slightly water-soluble or water-insoluble gases from chemical reaction.

◼

Water-insoluble gas is introduced into water of gas collection device and for its density of the gas is lower than that of the water, gas will rise to displace the water out. Oxygen (O2), nitrogen (N2) and hydrogen (H2) can be collected by this method.

◼

For gases that are soluble in water, such as ammonia (NH3), hydrogen chloride (HCl) …etc., they are not suitable to be collected by this method.

◼

For example:

➢

Invert the test tube that filled with water into the tub of water.

➢

Place the rubber tube into the opening of test tube that displace the water out.

➢

When the gas bubbles start to emerge from the opening of test tube, take rubber tube out of the test tube.

➢

Stand the test tube upside down on table (opening pointed downward) or stop it with a rubber stopper. T4 Video on YouTube (Click)

◼

A baked object need to go back to room temperature before weighing, the buoyancy of convective airflow around the pan will affect object’s apparent mass.

◼

Wipe balance clean with a damp cloth but do not use solvents.

◼

Do not invert balance upside down to avoid damage to the parts inside.

◼

Use folded paper, beaker or bottle as container while weighing, do not put chemicals on directly.

◼

Analytical balance is a more precise balance having maximum load of 210 g, close its windshields while zeroing and weighing or the reading is inaccurate.

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◼

Place balance on stable, non-vibrating table where no sun/wind, and temperature is stable.

◼

Do not overload the balance. Maximum load of an electric balance in lab is 610 g.

◼

Make sure balance is level, clean, and dry before use.

◼

Warm up the balance for 30 minutes before use.

◼

Put weighed object in the center of weighing pan.

T9 - Electronic Balance

Electric Balance (s.f. only contains two decimal places) Analytical Balance (s.f. contains four decimal places)

T9 Video on YouTube (Click)

◼ Use folded weighing paper or beaker to hold solid chemical. ◼ Take liquid chemical by public dropper. ◼ Use a clean spatula to take solid chemical. ◼ Keep the balance clean and control the amount of chemicals taking to

avoid excess weighing, dripping, and scattering around.

◼ Do not return excess chemicals to original bottle and put it in a waste

bottle.

◼ Keep bottle well closed after taking chemicals. ◼ Keep the balance clean at all times.

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T10 – Weighing Chemicals

T10 Video on YouTube (Click)