   Chapter 4, Problem 74PS

Chapter
Section
Textbook Problem

An unknown solid acid is either citric acid or tartaric acid. To determine which acid you have, you titrate a sample of the solid with aqueous NaOH and from this determine the molar mass of the unknown acid. The appropriate equations are as follows.Citric acid:H3C6H5O7(aq) + 3 NaOH(aq) → 3 H2O(l) + Na3C6H5O7(aq)Tartaric acid:H2C4H4O6(aq)+ 2 NaOH(aq) → 2 H2O(l) + Na2C4H4O6(aq)A 0.956-g sample requires 29.1 mL of 0.513 M NaOH to consume the acid completely. What is The unknown acid?

Interpretation Introduction

Interpretation:

The molar mass of acid used in the given reaction has to be determined and the acid is whether is citric or tartaric acid has to be identified.

Concept introduction:

• The molar mass of an element or compound is the mass in grams of 1 mole of that substance, and it is expressed in the unit of grams per mol (g/mol).
• For chemical reaction balanced chemical reaction equation written in accordance with the Law of conservation of mass.
• Law of conservation of mass states that for a reaction total mass of the reactant and product must be equal.
• Stoichiometric factor is a relationship between reactant and product which is obtained from the balanced chemical equation for a particular reaction.
• Concentrationofsubstance=Amountof substancevolumeofthesubstance
• Amountof substance=Concentrationofsubstance×volumeofthesubstance
• Titration is a method to determine the concentration of a substance in solution by adding a solution of known volume and concentration until the reaction is completed.
Explanation

Balanced chemical equation for the given reaction involving citric acid is,

H3C6H5O7(aq)+3NaOH(aq)3H2O(l)+Na3C6H5O7(aq)

From the balanced chemical equation it is clear that acid (H3C6H5O7H2A) and base (NaOH) react in a 1:3ratio.

The amount (moles) of NaOH used in the titration can be calculated from its volume and concentration as follows,

AmountofNaOH = CNaOH×VNaOH =0.513molL×0.0291L =0.0149molNaOH

Using the stoichiometric factor the amount of citric acid  H3C6H5O7 can be calculated.

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