Chemistry - Titration

bonkysleuth SG

Before I pose the question, I'd like to let you guys know the steps taken for this titration experiment.

Aim : To prepare a dry sample of sodium chloride by titrating dilute hydrochloric acid with aqueous sodium hydroxide.

1. Fill a burette with dilute hydrochloric acid.

2. Pipette 25.0cm3 (or 20.0cm3) of aqueous sodium hydroxide into a conical flask. titrate the alkali with dilute hydrochloric acid, using phenolphthalein. repeat the tiltration as many times as you think neccessary to obtain accrate resutls.

3. pipette 25cm3 (or20.0cm3) of aqueous sodium hydroxide into a conical flask. add appropriate volume of dilute hydrochloric acid (determined from titrations in step 2) into conical flask. this time, do not add the indication, phenolphthalein. swirl to ensure complete reaction. transfer the slution into an evaporating dish.

4. heat mixture from step3 to obtain saturated solution

5. allow the resulting saturated solution to cool slowly. filter to obtain the crystals formed.

6. dry the crystals obtained by pressing them between a few sheets of filter paper.

why was step 3 carried out without adding indicator??

my guess is that you've already known the volume of hydrochloric acid needed to titrate the aqueous sodium hydroxide and the repeated step is to ensure accuracy in volume of HCL added? i'm not sure how to continue from here, can someone help? thanks!

Darkness_hacker99 SG

Your question and your guess is contradicting leh. Maybe you typo bah.

Yar. Since you know it in step 2, so it's not really necessary to repeat the step(adding indicator) again.

That's my guess too.

weewee SG

phenolphthalein will react with H+ and OH- to form different chemical products which also gives different colour. These by-products may not evaporate even after heating.

If http://en.wikipedia.org/wiki/Phenolphthalein is not added, then you will just get NaCl (aq) and water. When you heat it, the water will evaporate and then you will get NaCl crystals.

bonkysleuth SG

phenolphthalein will react with H+ and OH- to form different chemical products which also gives different colour. These by-products may not evaporate even after heating.

If http://en.wikipedia.org/wiki/Phenolphthalein is not added, then you will just get NaCl (aq) and water. When you heat it, the water will evaporate and then you will get NaCl crystals.

So the indicator is simply used to gauge the acidity of the solution?

aiglosicicle SG

So the indicator is simply used to gauge the acidity of the solution?

indicator is used to find out exactly when you have a complete reaction, which means whatever there was inside the conical flask has been totally reacted away by whatever was in the burette. knowing the amount of acid and alkali used will be needed to find the pH of the solution yes yes there is a way to calculate it using the a'level syllabus.

if i can still remember... finding the weight of the crystals = finding number of moles of the salt formed. by comparing it against the amount of acid and alkali you used for step 2 (mole calculations) you can determine if your titration was accurate.

also, indicators arent completely neutral. they can and will affect the pH of the solution to a certain degree, which is why you only add a few drops of indicator for all titrations. if you want an accurate measurement of pH you shouldnt add indicator. but of course you still need indicator for step 2 first la otherwise you dont have the mole values for eventual calculation.

rusty already so i might not be right but hopefully makes sense.

UltimaOnline SG

What everyone (who replied TS/OP) said is correct, but weewee is closest to the point.

If the qn is why don’t add indicator the 2nd time, the direct answer is “to prevent contamination. you want the pure salt. not salt contaminated with indicator.”