# Chemistry

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.

1. 👍 0
2. 👎 0
3. 👁 178
asked by Rachel

## 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

asked by Sarah on October 24, 2013
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

asked by Nancy on August 2, 2012
3. ### 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

asked by Rob on February 6, 2007
4. ### 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

asked by Jazmin on May 15, 2011
5. ### 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

asked by kenzie on June 7, 2018
6. ### chemistry

If there are 80 milligrams of a radioactive element decays to 10 milligrams in 30 minutes then what is the elements half life in minutes? Ok i narrowed it down to either 10 or 30...but im rly confused so if u could help it wuld be

asked by jojo on March 4, 2007
7. ### 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

asked by John on November 25, 2006
8. ### Chemistry

For my homework the question is... What is the half life of polonium 214 if a 1 gram sample decays to 31.25mg after 820 seconds? Im not sure how you arrive at the answer, my friend says the answer is 5 but were not sure how to do

asked by jojo on March 4, 2007
9. ### math

The half life of the radio active substance is the time require the half the substance to decay. the mount A ( in grams) of 100 gram sample of a radio active substance remaining after t half lives by A equals to 100 (0.5) t.

asked by sai on January 17, 2018
10. ### Chemistry

Gallium65, a radioactive isotope of gallium, decays by first order-kinetics. The half-life of this isotope is 15.2 min. How long would it take for 7/8 of a sample of this isotope to decay? 15.2 min 30.4 min 45.6 min 48.0 min I'm

asked by Val on March 28, 2010

More Similar Questions