   Chapter 4, Problem 73PS

Chapter
Section
Textbook Problem

You have 0.954 g of an unknown acid, H2A, which reacts with NaOH according to the balanced equationH2A(aq) + 2 NaOH(aq) → Na2A(aq) + H2O(ℓ)If 36.04 mL of 0.509 M NaOH is required to titrate the acid to the second equivalence point, what is the molar mass of the acid?

Interpretation Introduction

Interpretation:

Molar mass of unknown acid, H2A used in the given titration process has to be determined.

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 is,

H2A(aq)+2NaOH(aq)Na2A(aq)+2H2O(l)

From the balanced chemical equation it is clear that acid (H2A) and base (NaOH) react in a 1:2ratio.

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

AmountofNaOH = CNaOH×VNaOH = 0.509molL×0.03604L = 0

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