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Question

A reaction is found to be 0th order with respect to thiosulfate ion. If the concentration of thiosulfate ion is 0.105M after 22.7 seconds have passed, and the rate law constant is 0.0187 M/sec, what was the initial concentration of thiosulfate ion?

Answer 1

Hi Aniya,

Thanks for your question. For a generic chemical species A that decomposes according to a zeroth-order rate law, we have the following equation:

[A] = -kt + [A]₀,

where [A] represents the concentration of A at time t, k is the rate constant, and [A]₀ is the initial concentration of A.

For this problem, we are given A is a thiosulfate ion and that [A] = 0.105 M at time t = 22.7 s. We are also given the rate constant k = 0.0187 M/s. We can plug all of these values into the equation above to get

0.105 M = -(0.0187 M/s)(22.7 s) + [A]₀.

This is now just an equation to solve for [A]₀, the initial concentration of our thiosulfate ion. Solving for [A]₀ to the correct number of significant digits, we find

[A]₀ = 0.529 M.