Sulfamic acid (97.094 amu) is a primary standard that can be used to standardize

Question

Sulfamic acid (97.094 amu) is a primary standard that can be used to standardize NaOH: +H3NSO3- + OH- ? H2NSO3- + H2O Standardization of a NaOH solution requires 34.26 mL of NaOH solution to react with 0.337 g of sulfamic acid. The monoprotic acid cacodylic acid (HC2H6AsO2, 137.9977 amu) was, at one time, a common herbicide. A 1.034 g sample of herbicide containing cacodylic acid and other inert compounds was titrated with the standardized NaOH solution. If the titrationrequired 8.47 mL of standardized NaOH solution to reach the equivalence point, what is the weight percent of cacodylic acid in the herbicide? Report your answer to three significant digits.

Explanation / Answer

Moles of sulphamic acid = 0.337/97.094 = 0.00347
Therefore molarity of standardized NaOH solution = 0.00347/0.03426 = 0.101 M

Let the weight of cacodylic acid be w gm
Moles of cacodylic acid = w/137.9977

At equivalence point, molarities are equal

0.101 = w/137.9977/0.00847
w = 0.101*137.9977*0.00847 = 0.118 gm

Therefore, Weight percent of cacodylic acid = (0.118/1.034)*100 = 11.4%

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