Lab Acids And Alkalis Sm

ACIDS AND ALKALIS INTRODUCTION During this month you have studied the general characteristics and reactions of acids and bases. You will test some of the reactions that have been studied in the class with three solutions (an acidic, a neutral and an alkaline solution respectively). You have also learned the meaning of neutralisation according to Arrhenius’ theory and discussed the possibility of assessing the acid or alkali contents of materials using adequate standard solutions and adding them drop-wise until an indicator (or a pH meter) shows that the sample has been neutralised. In the second part of this work you will use a neutralisation reaction to find the acid content of vinegar. AIMS:   

To identify an unknown solution as acid, neutral or alkaline To observe some characteristics of acidic, alkaline and neutral solutions To titrate the acid content of an unknown sample

1- TESTING ACIDITY 1- You are given three beakers labelled 1, 2 and 3 containing aqueous solutions, and some strips of blue and red litmus paper. 2- The samples are 10% solutions of HCl, NaOH and NaCl but have to be identified 3- Cut a red and a blue litmus paper in three parts. 4- Dip a different red and blue strip on each of the three solutions. 5- After 2 – 3 seconds check the colour of the strips. 6- Complete the following chart with your observations and conclusions: Sample Colour of red Colour of blue Identification ↓ litmus litmus 1 2 3 2- ESTIMATING pH 1- Cut the strip of Universal Indicator Paper (pH paper) in three parts. 2- Prepare three test tubes containing 10 ml of distilled water. Using a teat pipette (rinse it thrice after every test) add one drop of acid solution to the first tube. Shake until homogeneous. 3- Repeat point (3) with the second tube but using the alkaline solution and with the third one adding the neutral solution. 4- Add a drop of Universal Indicator to each test tube. Complete the chart below Observe the colours after 2 – 3 seconds and estimate pH for the three samples.

5- Dilute 1 ml of each test tube in a separate tube with water to the top and shake to homogeneity. 6- Re-test pH on all three samples after dilution 7- Complete the chart below. Colour pH Colour (dil) pH HCl NaOH NaCl 3- CHECKING SOME TYPICAL REACTIONS OF ACIDS AND ALKALIS 1- Fill three test tubes up to 2 cm from the bottom each with one of the acidic, alkaline and neutral solutions. 2- Add to each tube a tip of a spatula measure of sodium carbonate. Observe and record your results in the chart. 3- Discard the contents, rinse the tubes and repeat point (1) 4- Now add each tube with a strip of recently sandpapered aluminium foil. Observe and record your observations in the chart. Teacher will test the fizzing gas for you. This is a typical test for hydrogen gas. If there are no changes heat gently (teacher). 5- Once again discard, rinse and refill tubes as in (1) 6- Finally add a tip of a spatula measure of ammonium chloride to each of the tubes and observe. In case you don’t see any changes, smell the test tubes carefully. Record your results in the chart. Na2CO3 test Aluminium test (NH4)Cl test Acid solution Alkaline solution Neutral solution 4- WHAT IS THE ETHANOIC ACID CONTENT OF APPLE VINEGAR? Household vinegar is an approx. 5 % aqueous solution of acetic (ethanoic) acid. The amount of acetic acid in vinegar is one of the several quality controls that are usually carried out in vinegar manufacturing companies to comply with the marketing standards. To do this, a mass of vinegar is weighed and added with a standard solution of sodium hydroxide of known concentration. Phenolphtalein indicator shows the point at which all of the acid in the sample has been neutralised (end point) that is to say when the addition of alkali must be stopped. The volume of alkali at this point will be recorded and used to calculate how much acid was there in the sample (called “the analyte”). Aims

To determine the mass-to-volume percentage (% m/v) of acetic (ethanoic) acid in commercial apple vinegar. Pieces of apparatus and chemicals 5 ml graduated pipette and dispenser, 250 ml conical flask, wash bottle, 100 ml measuring cylinder, burette, retort stand, boss and clamp, funnel, standard NaOH solution, 1 % alcoholic phenolphthalein solution, CO2 free distilled water (recently boiled and cooled to room temperature). Procedure 1- Fix the titration apparatus as shown in the figure at the end of the instructions. Check that the stopcock is closed. 2- Using the funnel, fill the burette and level it at the zero point. The excess solution can be collected in any container (test tube, beaker) and discarded (sink). 3- Rinse the tip of the burette with water from the wash bottle. 4- Carefully pipette 5.0 ml of vinegar into the conical flask. The tip of the pipette must be touching the neck of the flask that will be tilted to 45º. 5- Rinse squirting a small amount of distilled water on the neck to wash down any rest of vinegar. 6- Add some 90- 100 ml of CO2 free distilled water. 7- Add 3 drops of indicator (phenolphthalein). 8- Now place the conical flask so that the tip of the burette is inside and just below its mouth. 9- Start adding drop-wise the alkali to the acid swirling the conical flask to mix. 10- When a drop turns the solution pink for an instant stop adding alkali. 11- Now add a drop at a time swirling and checking the colour of the solution. 12- When the pink colour does not fade on swirling, stop the titration. 13- Record the volume of sodium used. 14- Calculate the mass of neutralised acid. (1 ml of sodium hydroxide solution neutralises 6 mg of acetic (ethanoic) acid). 15- Calculate the concentration of acetic acid in vinegar as % m/v). 16- Write your results in the chart

Volume of vinegar (ml) Volume of NaOH (ml) Mass of acetic acid (g) % m/v

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