help me with titration question (1 Viewer)

[moniyoo]

hi all i was just doing these questions and i think i need a help im having difficulties giving a good answers.
this is the titration on white veneger with sodium hydroxide.

1.a) 'about 20ml of distilled water' is added to the venegar. why is it unneccessary to measure this volume accultely?
b) '2drops of phanophthelin' are added. why would a large amount of added phenophthalein increase the titration volume?

2. consumer laws rewuire vinegar to contain at least 4.0gL^-1 of acetic acid. (this question is hard..)

a) Doeds this brand of venegar meet this requirement?
b) Suppose an unscorpulous manufacturer had added some sulfuric acid to increase the acidity of the vinegar. How would you test for presence of sulfuric acid in cinegar?


and why different indicator gave vey diff results in the titration?

 

[nightweaver066]

1. a) The moles of acetic acid in your sample of vinegar you'll be using remains the same. Increasing the volume will not affect the moles and will not affect how much sodium hydroxide you need to add.

b) Indicators are weak acids (can also be weak bases) that form equilibrium reactions in solution. When more phenolphthalein is added, there is an increase in hydrogen ions in solution and as as result, more NaOH is required to neutralise all the hydrogen ions in the analyte.

2. a) you're missing info
b) This is a chemical monitoring and management question. Add barium nitrate and obseve for a white precipitate. Add lead sulfate and observe for a white precipitate.

 

[John Rips]

regarding Q2. (a) You need to calculate the molarity of the acetic acid in the original sample from the results of your titration (or supplied data) which is missing in your question, convert that to grams per mL and assess if it meets the standards. (b) With regards to the sulphuric acid question, I'm assuming that you are titrating with a strong base. Sulphuric acid is dipriotic, so would need twice as much base as an equivalent molar solution of acetic acid. This is one method but means you need to make a solution of acetic acid once you know the pH of your vinegar. A second method could be if you noted a few of the pH changes during the titration (pH meter or god forbid, pH paper) or especially measured the pH at the equivalence point that would indicate whether you have a strong or a weak acid in your solution. The above two answers are within the confines of The Acidic Environment. In a proper lab, then you could also add barium acetate or nitrate to a small sample of your vinegar solution and look for a white precipitate of BaSO4, but you would need a control sample of non-polluted vinegar to ensure normal untainted vinegar doesn't contain sulphate ions.
With regards to different indicators, you need to select an indicator that changes colour near to the end point pH. Stong acid + strong base would use bromothymol blue whereas here its phenolpthalein, and for a weak base + strong acid would use methyl red.