25.0 cm³ of vinegar diluted tenfold needs 20.80 cm³ of 0.100 mol/dm³ NaOH. What is the concentration of the original vinegar?

The original vinegar is 0.832 mol/dm³ ethanoic acid. The working is 0.02080 × 0.100 = 0.00208 mol, then 0.00208 ÷ 0.0250 = 0.0832 mol/dm³, then × 10 = 0.832 mol/dm³.

Convert 0.832 mol/dm³ ethanoic acid into grams per dm³.

The concentration is about 49.9 g/dm³. The working is 0.832 × 60.0 = 49.9 g/dm³, using Mr = 60.0.

Why can an indigestion tablet not be titrated directly with acid?

The tablet is an insoluble solid that reacts slowly, so the end point is hard to judge. A back titration is used instead.

What is a back titration?

A back titration adds a known excess of reagent to the sample and then titrates the unreacted excess. The amount that reacted with the sample is found by difference.

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Analytical Chemistry Techniques

Applied titrations of vinegar, indigestion tablets and bleach

3 things to know
A2 9

Overview

Titrating vinegar with sodium hydroxide, back titration of indigestion tablets, iodometric titration of bleach with thiosulfate and starch, with equations and worked concentrations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

25.0 cm³ of diluted bleach releases iodine that needs 18.00 cm³ of 0.100 mol/dm³ thiosulfate. What is the chlorate(I) concentration?

Key term

Write the equation for the reaction of ethanoic acid with sodium hydroxide.

Key term

Why is vinegar usually diluted before it is titrated?

Questions from this subtopic

Each one with the model answer underneath.

1
Write the equation for the reaction of ethanoic acid with sodium hydroxide.
Model answer
CH₃COOH + NaOH → CH₃COONa + H₂O. One mole of ethanoic acid reacts with one mole of sodium hydroxide.
2
Why is vinegar usually diluted before it is titrated?
Model answer
Vinegar is fairly concentrated, so undiluted samples would need very large titres. A tenfold dilution in a volumetric flask gives a titre of a convenient size.
3
25.0 cm³ of vinegar diluted tenfold needs 20.80 cm³ of 0.100 mol/dm³ NaOH. What is the concentration of the original vinegar?
Model answer
The original vinegar is 0.832 mol/dm³ ethanoic acid. The working is 0.02080 × 0.100 = 0.00208 mol, then 0.00208 ÷ 0.0250 = 0.0832 mol/dm³, then × 10 = 0.832 mol/dm³.
4
Convert 0.832 mol/dm³ ethanoic acid into grams per dm³.
Model answer
The concentration is about 49.9 g/dm³. The working is 0.832 × 60.0 = 49.9 g/dm³, using Mr = 60.0.
5
Why can an indigestion tablet not be titrated directly with acid?
Model answer
The tablet is an insoluble solid that reacts slowly, so the end point is hard to judge. A back titration is used instead.
6
What is a back titration?
Model answer
A back titration adds a known excess of reagent to the sample and then titrates the unreacted excess. The amount that reacted with the sample is found by difference.

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