Basic Math Question

[ga14]

Hey Guys,

Cramming for finals, and here's a problem I got wrong on one of my old exams that's pretty easy, but I just can't do!

"A reaction follows the equation:

t=Aexp(Q/RT) where t is in secs, Q the activation enegery, R the gas constant (1.98 cal/mol-K), an T is the absolute temperature. If it takes a substance 0.1s to start forming at 600 degrees C, an at 500 degrees C it starts after 10 s, how long will one have to wait for the substance to form at 400 degrees C?"

If someone could show me the steps, that would be great.

I started doing the following and got two equations:

(1) ln 0.1=ln A - Q/R(873)
(2) ln 10= ln A - Q/R(773)

But now what? Do you take the ratio of the two? Then what? Thanks for any help!

 

[ClueLis]

ln(.1)=-ln(10)

Just a thought.

 

[TuxDave]

Originally posted by: ga14
Hey Guys,

Cramming for finals, and here's a problem I got wrong on one of my old exams that's pretty easy, but I just can't do!

"A reaction follows the equation:

t=Aexp(Q/RT) where t is in secs, Q the activation enegery, R the gas constant (1.98 cal/mol-K), an T is the absolute temperature. If it takes a substance 0.1s to start forming at 600 degrees C, an at 500 degrees C it starts after 10 s, how long will one have to wait for the substance to form at 400 degrees C?"

If someone could show me the steps, that would be great.

I started doing the following and got two equations:

(1) ln 0.1=ln A - Q/R(873)
(2) ln 10= ln A - Q/R(773)

But now what? Do you take the ratio of the two? Then what? Thanks for any help!

Two equations, two unknowns (A and Q), what's the problem? You can use any method to solve. You can take the top equation and subtract the bottom equation to isolate the Q?

 

[bleeb]

if you got that wrong, then you're hopeless and it would be pointless to try to help with this basic math question. good luck and may God have mercy on your soul. 😛

 

[Martin]

e^ln (x) = x

 

[ga14]

Originally posted by: bleeb
if you got that wrong, then you're hopeless and it would be pointless to try to help with this basic math question. good luck and may God have mercy on your soul. 😛

sure sure...you can't do it...admit it ;-)

but yes...this was easy and i was dumb for missing 5 points because of it. 😱

 

[Legendary]

solve for Q in one, substitute into the other.

 

[bleeb]

Originally posted by: ga14

Originally posted by: bleeb
if you got that wrong, then you're hopeless and it would be pointless to try to help with this basic math question. good luck and may God have mercy on your soul. 😛

sure sure...you can't do it...admit it ;-)

but yes...this was easy and i was dumb for missing 5 points because of it. 😱

hahahah I'm a math major guy! hehe I'm just playin though =) I'm currently sick so don't have the mental capacity to think. Flu bug got me..

 

[Zorba]

Solve one equation for Q and plug that into the other equation to find A. Then find Q. Then do your equations at 400ºC and get the answer right, easy.