[LHM]

The reaction A+B AB is 1st order with respect to A and zero order to B. The reaction is begun with the initial concentration of both reactants at 0.100 M. After 1.5 hours the concentration of B has dropped to 0.060 M. What is the approximate value of the specific rate (reaction rate) constant for this reaction?
A) 0.15 hr^-1
B) 0.34 hr^-1
C) 0.61 hr^-1
D) the specific rate constant cannot be determined unless the mechanism of the reaction known.

Here's what I tried:
The rate=k[A]¹[ B ]⁰, and [A]=0.100 M. For the rate, I just subtracted 0.060 M from 0.100 M for the  :delta: [ B ], and then divided that by 1.5 hours to get 0.0267 M/hr. The equation then becomes 0.0267 M/hr=k*(0.100)(1), which gives k= 0.267 hr^-1, which isn't a choice, so what did I do wrong?

[MrTeo]

The equation then becomes 0.0267 M/hr=k*(0.100)(1)

On one side you have the medium reaction rate (0.0267 mol∙L^-1∙hr^-1) while on the other the maximum reaction rate (at the beginning of the reaction, when the concentration of A is at its maximum).

[igloo5080]

Your calculation of rate is an average one taken over 1.5 hours, so your answer is invalid.

Since A and B both start with same concentration, and that they both react in a 1:1 molar ratio (according to the equation) when the concentration of B drops from 0.100M to 0.060M, so does that of A

Next you need to use the standard equation for 1st order kinetics, which is  ln(c₀/c_t) = k x t

This will lead to a value for k which matches one of the options given.

[LHM]

Your calculation of rate is an average one taken over 1.5 hours, so your answer is invalid.

Since A and B both start with same concentration, and that they both react in a 1:1 molar ratio (according to the equation) when the concentration of B drops from 0.100M to 0.060M, so does that of A

Next you need to use the standard equation for 1st order kinetics, which is  ln(c₀/c_t) = k x t

This will lead to a value for k which matches one of the options given.

Thanks. However, when I use ln(0.1/0.06) = k x 1.5 hr, I get that k=0.34 hr^-1. This is answer choice B, but the answer is C. Am I doing something else wrong?

[MrTeo]

Actually I get 0.34 hr^-1 too, even using another method to solve it... that's quite strange...