# AS Carboxylic Acids Evaluative Help please!

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#1
Hi, I'm resitting my AS eval very soon (1-2 days) and want to just quickly check some of these answers to the questions from this practise sheet I've been given please as I suck at these!

Carboxylic acids: Structure 1= CH3CH2CH=CHCH2COOH
Structure 2= CH3CHOHCH=CHCH2COOH
Structure 3= HOOCCH2CH2COOH

1) The student then pipettes 25cm3 of 0.150moldm-3 potassium hydroxide into a conical flask and puts some of the carboxylic acid solution in a burette. The student carries out a titration using the 2 solutions. What volume of carboxylic acid solution would need to be added from the burette to completely neutralise the potassium hydroxide?

I don't need help with this one, just included it as you need it for the next question. I got it as 18.75 cm3, which is for carboxylic acid structure 1 (CH3CH2CH=CHCH2COOH).

2) Another student carries out the same experiment but uses the carboxylic acid with structure 3. He makes up 250cm3 of a 0.200moldm-3 solution of the carboxylic acid and pipettes 25cm3 of 0.150moldm-3 potassium hydroxide into a conical flask. He puts some of his carboxylic acid solution in a burette and carries out a titration using the 2 solutions. Predict, explaining your answer, what volume of this carboxylic acid that would be required to completely neturalise the potassium hydroxide. (Hint: There's no need to a long calculation, use your answer to 1).

I put 18.75/2= 9.375cm3, as there are 2 COOH groups in the one molecule compared to structure 1, so 2 KOH is needed. So the equation would be HOOCCH2CH2COOH + 2KOH (as opposed to 1 KOH), therefore there's a 1:2 ratio between the carboxylic acid and KOH (as opposed to 1:1 with structure 1).

3) A student carries out a titration of a carboxylic acid and sodium hydroxide. When making up a solution of sodium hydroxide she added too much water to the volumetric flask and mixed it thoroughly. She removed the extra volume and carried on with the titration as normal. Would this have increased or decreased the titre volume of the carboxylic acid obtained from the burette?
I put "Decreased-the no. moles of NaOH would be smaller as the concentration would stay the same but the volume would decrease. So when using the molar ratio to find out the no. moles of carboxylic acid, that also decreases, so the volume decreases.

4) Another student carries out the same titration but makes up the solution correctly. He washes out the burette with water but forgets to watch out the burette with carboxylic acid. He fills the burette with the carboxylic acid and carries out the titration. Would this error have increased or decreased the titre volume of the carboxylic acid that would be obtained from the burette?

I didn't really know with this one D; Would it increase it slightly since there's more carboxylic left in the burette which gets added to it to the next time?

5) Yet another student carries out the same titration, makes up the sodium hydroxide solution correctly and washed all the apparatus correctly. He washes the conical flask with water and then continues the titration with the wet flask. Will this affect the volume of acid measured from the burette?

I put 'Yes because the added water will add to the NaOH, making it more dilute so that less volume of the acid in the burette will be needed to neutralise it.'

Thank you very much
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7 years ago
#2
Hi there,

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