I know how to find half-lives, but I have trouble finding the length of time it would take for the concrentration of X to decrease to Y% of its initial value.

For example, this is one of the problems:

Methyl isocyanide, CH3NC, isomerizes when heated to give acetonitrile (methyl cyanide), CH3CN.

CH3NC(g) ---> CH3CN(g)

The reaction is first order. At 230 degrees, the rate constant for the isomerization is 6.3 x 10-4/s. What is the half-life? How long would it take for the concentration of CH3NC to decrease to 75.0% of its initial value? to 30.0% of its initial value?

I’m not sure if this one would work the same way, but I needed help with this problem as well. If they are done the same way, could you do at least a couple from this 2nd problem when it starts asking for the 25% and 12.5%?

In the presence of excess thiocyanate ion, SCN-, the following reaction is first order in chromium (III) ion, Cr3+; the rate constant is 2.0 x 10-6/s.

What is the half-life in hours? How many hours would be required for the initial concentration of Cr3+ to decrease to each of the following values: 25.0% left, 12.5% left, 6.25% left, 3.125% left?

Any help would be greatly appreciated!!! Thanks!

P.S. Sorry, I couldn't do superscripts on here!

Have you tried t1/2=0.693/k to obtain half-life?

Have you tried, for the second part of the first problem, assuming you start with 100 molecules (No) of CH3N?. For it to decrease to 75% of its original value means N=75 and plug those values into ln(No/N)=kt and solve for t? Check my thinking.

asked by Rachel

Respond to this Question

First Name

Your Response

Similar Questions

  1. physics

    If the physical half-life of sulfur-35 is 87.1 days, and the biologic half-life in the testicle is 632 days, what is its effective half-life in this organ? I know i have to find the time and then the half-life. But im having
  2. chemistry

    Find the number of disintegrations per minute emitted by 1.1 mol of 232-Th in 1 min. The half-life of 232-Th is 1.4 x 1010 year I was able to do this with the following technique: 1.1 x 6.022 x10^23 =6.62x10^23
  3. chemistry

    Butadiene, C4H6, in the gas phase undergoes a reaction to produce C8H12. The following data were obtained for the reaction. [C4H6] Time (min) 1 0 0.98 20 0.91 100 0.66 500 0.49 1000 0.32 2000 0.25 3000 Determine the order of the
  4. Physics

    How many half-lives are required for the number of radioactive nuclei to decrease to 1/10^4 of the initial number?
  5. chemistry

    If, for a second-order reaction, the elapsed time after the first half-life is 338.871 seconds, the elapsed time after four half-lives would be (in minutes)... t1/2= 1/k(A). k= 1/t1/2*(A)=1/338.871 s(1) (I let (A)=1). k = ?? After
  6. Chemistry

    K-42 has a half-life of 12.4 hours. If you begin with 250g of K-42, determine how much you will have left after 4 half-lives. How long does 4 half-lives take? please help
  7. chemistry

    Some radioactive nuclides have very short half-lives, for example, I-31 has a half-life of approximately 8 days. Pu-234, by comparison has a half-life of 24,000 years. Explain why both of these examples are dangerous, even though
  8. Chemistry

    How many half-lives have passed if 87.5% of a substance has decomposed? How many if 99.999% has decomposed? For 87.5% decomposition 1/8 is left. (1/8) = (1/2)^3 Three half lives is the answer Fot 99.999% decomposition, 10^-5 is
  9. hard math question

    When a certain drug enters the blood stream, its potency decreases exponentially with a half life of 6 hours. Suppose the initial amount of the drug present is A (sub 0). How much of the drug will be present after each number of
  10. Chemistry

    This problem is 1st order for half life. How long would it take for the concentration of cyclopropane to decrease to 50% of its initial value? to 25% of its initial value? Rate constant is 9.2/s I set up like this... t1/2=

More Similar Questions