Chapter 4.9, Problem 3.1ACP

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# Calculate the chloride ion concentration in a sample of sea-water given the following experimental information: The volume of original seawater sample was 100.0 mL. A 10.00-mL sample of the seawater was diluted to 100.0 mL with distilled water, and 10.00 mL of the diluted sample was again diluted to 100.0 mL. A Mohr titration was performed on 50.00 mL from the second dilution, and this sample required 26.25 mL of 0.100 M AgNO3 to reach the equivalence point.

Interpretation Introduction

Interpretation:

The chloride ion concentration in a given sample of sea water has to be determined.

Concept introduction:

• The relation between the number of moles and mass of the substance is ,

Numberofmole=MassingramMolarmass

Massingramofthesubstance=Numberofmole×Molarmass

• 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.
• Mohr method is a common method used to analyze a solution for its chloride ion content. In this method a solution containing chloride ions is titrated with standardized silver nitrate and the following reaction will occur,

Ag(aq)++Cl(aq)AgCl(s)

Explanation

The balanced net ionic equation for the reaction occurring in the Mohr method is,

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  Ag(aq)+â€‰+â€‰Cl(aq)âˆ’â€‰â€‰â†’â€‰AgCl(s)

Amount of Ag+ can be determined as follows,

Â Â Â Â Â  Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â AmountÂ ofâ€‰Ag+â€‰=â€‰CAg+â€‰Ã—â€‰VAg+

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  =â€‰0.100â€‰molâ€‰Ag+1Lâ€‰Ã—â€‰0.02625â€‰L

Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  =â€‰2.625Ã—â€‰10-03â€‰molâ€‰Ag+

From the amount of Ag+ number of moles of Clâˆ’ can be calculated. Stoichiometric ratio between Ag+ and Clâˆ’ is 1:1

=â€‰Â 2.625Ã—â€‰10-03â€‰molâ€‰Ag+â€‰Ã—â€‰1â€‰molâ€‰Cl-1â€‰molâ€‰Ag+â€‰=â€‰2

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