1. A first order reaction is seen to go from a reactant concentration of 0.0451

Question

1. A first order reaction is seen to go from a reactant concentration of 0.0451 M to 0.0321 M in 41.8 seconds? How long (in seconds) after the reaction starts will the reactant concentration be 0.0180 M? I know the correct answer to this is 113 seconds, but I'm not sure how to get there.

2. HCOOH (g) CO2 (g) + H2 (g)

At 555.0° C, the half life for this reaction is 23.5 min. How many seconds are needed for the initial concentration of HCOOH to decrease by a factor of 10? The correct answer to this is 4740 seconds, but again not sure how/why.

Explanation / Answer

The decomposition of HCOOH is first order.

Half life= 0.693/ k

K= 0.693/ 23.5 =0.029489 mins-1

Since the concentration has to decrease by a factor 10

1-C/CO= 0.9

C/CO= 0.1

-ln(1-XA)= Kt

-ln(0.1)= 0.029489*t and hence t =2.30259/0.029849 = 78.1 minutes= 78.1*60 seconds =4686 seconds

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