Consumption rate of a reactant

Hello,

I have a doubt in an exercise about rate of reaction. The statement is quoted above and the reaction scheme is the following:


As the reaction is second order, I tried to apply the formula r = k*C^2.

Where r = rate of reaction
k = rate constant
C = concentration

So, I got:
r = (7.7e-3)*(3,091)^2 = 0,073 M

However, this not correct. I know it because when I click the "Send answer" button, it says "incorrect". Furthermore, the "Show answer" button doesn't display the answer.

Could anyone give some help?

Thanks a lot.

guiromero said:

So, I got:
r = (7.7e-3)*(3,091)^2 = 0,073 M

Is this result properly rounded? (Also, careful with the units.)

I tried to round from 0.07 to 0.075 and it didn't work.
The units are ok, as k[M^-1*s^-1] * C^2[M^2] = r[M * s^-1]

guiromero said:

I tried to round from 0.07 to 0.075 and it didn't work.

In my calculator,
$$
7.7 \times 10^{-3} (3.091)^2 = 0.073567963
$$
which, rounding to two significant figures, should give 0.074. You might try also 0.0736 or 0.07357 in case the preprogrammed solution was entered with an incorrect number of significant figures.

guiromero said:

The units are ok, as k[M^-1*s^-1] * C^2[M^2] = r[M * s^-1]

It's just that you wrote the units as M instead of M/s in the OP.

DrClaude said:

In my calculator,
$$
7.7 \times 10^{-3} (3.091)^2 = 0.073567963
$$
which, rounding to two significant figures, should give 0.074. You might try also 0.0736 or 0.07357 in case the preprogrammed solution was entered with an incorrect number of significant figures.It's just that you wrote the units as M instead of M/s in the OP.

I tried the values that you mentioned but it didn't work.
I think what you trying to say is the units of rate. I put M*s^-1 for M/s. I don't know if this is what you mean.

guiromero said:

So, I got:
r = (7.7e-3)*(3,091)^2 = 0,073 M

You wrote M instead of M s^-1.

DrClaude said:

You wrote M instead of M s^-1.

Oh yes, thanks.