How do you determine percent composition of a mixture of sugar and salt using conductivity?

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t-nez | High School Teacher | (Level 3) Associate Educator

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The conductivity of a solution is proportional to the number of ions it contains. Sodium chloride (table salt) ionizes in water and sugar does not. If you have a conductivity meter you can find the conductivity of the solution, use that to determine the amount of salt present, and the rest will be sugar.

You will need to know:

- the mass of the undissolved mixture

- the volume of the solution

- the conductivity of the solution

- the molar conductivity of sodium chloride, calculated below:

Molar conductivity of NaCl=(molar conductivity of Na+)+(molar conductivity of Cl-)

since there's a 1: ratio of Na+ to Cl-,

molar conductivity= (0.05011)+(0.07635) = 0.12646 S x L x mol-1

S is Siemens, which is the inverse of Ohms.

0.12646 x molarity of ions = conductivity measured, 

so molarity = conductivity/(0.12646)

in moles/liter

(molarity of ions)(liters of solution) = moles of NaCl

Moles of NaCl x 58.5g/mol  

= grams NaCl

(grams NaCl)/mass of mixture x 100% = % NaCl

100-%NaCl = % sucrose (sugar)

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