Here's the first one:

The half-life for the radioactive C-14 is 5730 years. How long will it take for 25% of the C-14 atoms in a sample to decay? If a sample of C-14 initially contains 1.5 mmol of C-14, how many millimoles will be left after 2255 years?

Okay, so I think I figured the first part. Since the half life is 5730. Half of that would be 25% making it 2865 years. As for the second part, I'm not sure where to start. It does not give anything about whether it is zero, first or second order so I can't use an equation. Am I missing something? The other thought I had was to maybe just do unit conversion but that seems too easy for this problem. I think it has to involve the half-life expressions.

Lastly, there is another problem I am struggling with. It gives the equation: CHCl3 + Cl2 => CCl4 + HCl

It says: The reaction is first order in CHCl3 and one-half order in Cl2, Which of the following reaction mixtures would you expect to have the fastest initial rate?

3 Choices:

a. 3 CHCl3 molecules, 3 Cl2 molecules

b. 4 CHCl3 molecules, 2 Cl2 molecules

c. 2 CHCl3 molecules, 4 Cl2 molecules

I think it is b because 1st order reactions are faster than 1/2 order reactions- I think. However, the book and my professor has not talked much about 1/2 order reactions so that is what is confusing. I know if it was first and second order, it would be b for sure because second order would be fast but I'm not sure. Any help would be much appreciated!

Thank you!!