Molecular formula of starch can be represented as (C₆H₁₀O₅)ₙ. If the gold number of one such starch is 6.48 and 0.01 millimoles of this starch is required to be added in 10 ml of red gold sol to just

Question

Molecular formula of starch can be represented as (C₆H₁₀O₅)ₙ. If the gold number of one such starch is 6.48 and 0.01 millimoles of this starch is required to be added in 10 ml of red gold sol to just prevent coagulation by adding 1 ml of 10% sodium chloride solution, then the value of 'n' is

Prashant Kotian · Accepted Answer

Correct answer: 4. Gold number is the amount of protective colloid in milligrams needed to prevent coagulation of 10 ml of standard gold sol by 1 ml of 10% NaCl. Thus, mass of starch required = 6.48 mg = 6.48 × 10⁻³ g. Moles of starch = 0.01 mmol = 10⁻⁵ mol. Molar mass of starch = Mass / Moles = (6.48 × 10⁻³ g) / 10⁻⁵ mol = 648 g/mol. The molar mass of the repeating unit (C₆H₁₀O₅) is (12×6 + 1×10 + 16×5) = 162 g/mol. Therefore, 162n = 648, which gives n = 4. Therefore, correct answer is 4.

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