Confidence Interval; Standard deviation.

[dylanbui]

1. The standard deviation of the lifetimes of a sample of 200 electric light bulbs was computed to be 100 hours. Find (a) 95% and (b) 99% confidence intervals for the standard deviation of all such light bulbs.

2.How large a sample of the light bulbs in the previous question must we taken in order to be 99.73% confident that thw true population standard deviation will not differ from the sample standard deviation by more than a). 5% b). 10%

For the first equation, I know how to use the equation, but my chi square table doesn't have values for the df. I suspect that I need to use a different equation because the sample size is too large, but I can't find any equations.
As for the second question, I'm totally stuck.